[MENTION=6691663]Tobold[/MENTION]: Even if adventure 1 starts at level 3 as written, it should be pretty easy to introduce some graduated steps within it, and start at level 1 anyway.
For example, there is an introductory encounter right at the start of the first adventure that should be very easily to scale down to level 1 (it kinda doesn't matter what level you are for it), and even if for some reason something went terribly wrong with the dice, it's in a public area and the PCs could be assisted or bailed out by nearby NPCs.
Then there's (an amazing!) set piece that happens soonish after that, where you could run it as level 2, but the context (stopping saboteurs aboard a ship) makes it easy to add in friendly henchmen to make up for the PC's weakness, either from the get-go, or as a backup if things go south.
After *that*, the next major arc of the plot involves deploying the PCs to a remote location (invade an enemy-held island), and you would want them properly leveled before they go on that mission, as they are now more properly "on their own." It's symbolically nice to divvy it up that way too.
That way you're still running over half of the adventure at the prescribed level (3), but the first half lends itself well to being the "intro adventure" you were looking to run anyway.
Hope that helps! (Please note my observations are based on running the Pathfinder version of said adventure, and I'm just a fan, and not privy to how they might be changing it up for 5e, so take with a grain of salt.)