"One important note. In this case, I'd probably avoid the 3 failures rule... it's hard to imagine the players actions making their predicament worse...which is what the 3 failures rule emulates best."The basic premise of creating a structured way to handle a non-combat challenge is totally sound.
However, the execution is not. The basic maths don't work, as expressed upthread. But more importantly, the premise may be flawed.
My basic rules of design for using "skill" challenges in my games are threefold:
- Understand the situation fully, determine whether a "skill" challenge is the best method to handle it, and begin by defining the stakes & consequences for failure.
- There is no one size fits all "skill" challenge. Instead thing about the specifics of your scenario and design toward that.
- Don't fixate on the "skill" part of "skill" challenges. Instead focus on creating holistic scenarios which can be resolved through multiple avenues including roleplaying, expending other resources (spell, hit dice, class features, Inspiration), specialized gear, creative ideas, etc. This is why I put the "skill" part in parentheses.
You can see a recent example I crafted for an upcoming session in my Tomb of Annihilation campaign: https://forum.rpg.net/showthread.php?834513-5e-Help-with-a-travel-skill-challenge
To apply these principles to your specific scenario.....
The Exploding Dungeon
Step 1: Knowing nothing beyond "exploding dungeon", I'd suggest clarifying the stakes first. Do you want it to be possible for a PC to die? All the PCs? I'd consider how much of the "exploding dungeon" was coming from what I wanted to introduce vs. consequences to decisions the players previously made. And I'd consider how much foreshadowing of the possibility for the explosion I'd done. IF this were a consequences for a previous decision where things went pear-shaped AND I'd done plenty of foreshadowing... only then would I consider death as a possible consequence.
Alternative consequences for failing the challenge completely might include the party getting separated, losing a NPC henchman/companion or an animal companion/familiar, suffering an enduring wound that only heals with downtime or the regeneration spell, losing precious items like spellbooks, becoming trapped in a very deadly dungeon that collapses on top of them, getting blasted by a magical explosion into another plane, losing Hit Dice prior to a dungeon where long resting isn't possible...
After deciding that, I'd consider how much "screen time" should be devoted to this challenge. Could it feasibly be resolved with a single action/check by a player in the party...for example, a high-level PC casting teleport or Otiluke's resilient sphere? Then it's handled and done. Or is it a small dungeon with the players rushing to get out and reach a rival who stole the Ruby of Xalmectur, likely leading to a new scene? In that case, a group Athletics check or a Constitution save might be sufficient, applying some damage and/or delaying those PCs affected before the combat with their rival.
Assuming that I think it merits a more detailed scene with meaningful choices for the players, I'd then proceed to the next step..
Step 2: Now you want to devise the mechanical structure for the challenge. I like to involve all the players, and one way of doing that is requiring each of them to engage with the challenge in some way. Another option might be to establish stages with stakes at each stage – for example, a collapsing stone bridge, a passage of collapsing statues, and a narrow squeeze to the exit. Another option would be a "time limit" expressed by placing a maximum limit on the number of actions/checks could work before the explosion.
One important note. In this case, I'd probably avoid the 3 failures rule... it's hard to imagine the players actions making their predicament worse...which is what the 3 failures rule emulates best.
All of this goes back to the narrative. "An exploding dungeon." Ok, what else? What specifically is causing this dungeon to explode? Is this a fiery gunpowder explosion? A collapsing mine? What sort of structural elements would fail and how do you imagine them failing? How are the dungeon inhabitants likely to respond?
And where does the conflict lie? Maybe the conflict isn't "can they escape in time"... Maybe the main conflict is getting chased by a monster while cave-in or explosion hazards are secondary? Maybe the conflict is "do they escape with the McGuffin or is it lost in the mad dash out, falling down into the darkness below"?
Step 3: The last step is imagining potential solutions to the extent that those can help you adjust your design toward the capabilities of the players and their PCs. You don't want lists of "accepted" or "likely" skills here (e.g. DC 15 Perception, Athletics, Acrobatics, Survival, Nature). That's the player's job. You want to detail challenges. For your scenario these might things like (1) evading falling debris, (2) ascending a sloped floor, (3) crossing crumbling stone stairs, (4) toppling statues, (5) erupting pockets of gas, and (6) a water-filled escape tunnel.
What's important to consider at this stage is are whether the challenges suit your party & the stakes you've set. For example, with #2 and #3, are you prepared to let PCs failing here fall down into the darkness and suffer those consequences (e.g. separation from the party)? Alternately, #6 would be a non-risk for a party of amphibious or otherwise water-breathing PCs.
And that's my basic approach!
Given the stage that you mentioned this specifically called out collapsing bridges, passages with collapsing statues etc, you really have a hard time getting to see a failure at one of those as making matters worse?
How about "your attempt to cross the collapsing bridge failed and now you have fallen back down two levels that you have already had yo work up from once.
For scenarios like this I often recall the Poseidon Adventure and while yes it plays as a slow motion race to avoid the blowing up dungeon, it's pretty good at showing consequences and outcomes.
Another take on this grew out of an encounter I had in a game...
The pcs were heading to a raider base to free captives and stop the raiders. On the way by random luck, thryvencountered a group that included the raider leader, big boss, and that surprising encounter was fantastic. But what happened after even more. When the raiders realized the approaching opposition took out the captain, things broke down and different folks started going for different objectives. The pcs had to rush in and deal with the fact that some were trying to grab stuff and run others looking to take over, others wondering about killing the hostages and witnesses and running, etc etc etc.
So you had a tired and banged up group facing not a case of infiltrate thru organized resistance but trying to still obtain several key objectives in a mess of conflicting choases.
While not strictly resolved as a skill challenge check off, it could have been and in fact many of the same elements were at play - social checks to rouse allies quick helped, tech skills to hack into security to see which were more pressing time wise, combat tasks to stop certain factions, stealth etc etc etc.