You can be anywhere from still level 1, to just turned level 3, by the time you wake up on the beach (in my test saves, I usually keep a save that is 'just before doing the level 3 level up' on the beach before I decide whether I'm getting shadow or not), and that makes a big difference, of course... However, each of the intellect devourers should, on average, be taken out by any damaging first level spell - guiding bolt will kill any brain the vast majority of the time, and whatever 1st level offence you picked for your caster will more or less do the same. If they hit you, they'll hurt, and may even down a 1st level character easy enough, but if they have to dash then they're not going to be attacking that round anyway, and even at disadvantage from being threatened, your odds of hitting them with an attack roll using your casting ability are high. If you sneak when the game warns you to be careful, then your first spell strikes should be almost guaranteed hits, and remove two of the three targets right away.
