Also, instead of direct threats to the PCs, think of indirect threats.
1. Things that the PCs want to protect. NPCs, objects, whatever.
Hostages, citizens, whatever. Good old rampage. Roll dice to see how many civilians die.
2. Enemies that are buying time for something bad to happen elsewhere.
Maybe a BBEG is running away, or executing their plot. This fight is chaff, and so is the next one.
3. PCs buying time for something good to happen elsewhere (or even here, like reinforcements), up against an escalating and infinite horde of foes.
Start with 50 zombie points. Each round roll 2d6 and add to the zombie points; if you roll doubles, roll again ("explode"). Whenever you roll a 6, a wave of zombies arrive equal to 1 for every 10 zombie points (round down).
Wave size is 5 + .82 per round.
On average 0.39 waves/round.
They can't stand against it forever. The longer they last, the more refugees escape.
4. Crazy amounts of enemies the PCs can engage whenever they want, but the foes are on the move towards targets.
A tarrasque, a pit fiend with dozens of devils, 1000s of bezerkers, a naval invasion with 100+ ships, a dracolich with a kobold cult, and a dozen mind flayers are all going to attack a city in various ways. Their attacks arrive in 5-10 days. What do you do?
In every case, you don't need a "fair" fight capable of dropping a PC every round for there to be risk and danger. This is T3, they can pick their fights. Odds are they can beat a number of the above, the question becomes how efficiently can they do it?