Ahtlon

Well Corsair gives you the option to do it, but it's a option, best way is 2 hhd with one empty bay open and place the third one some where else in the case , like on the bottom of the case, there is room enough in front of the psu, you could use the hdd plastic holder of the cage, put the hdd in it and fix the holder with hdd with some velcro on the bottom.

I never install 3 hhd in one cage, I always let one place open between the hdd, because if there isn't a good airflow, they will heat up each other. You could remove the middle hdd and place it some where else, for fixing this one you could use some Velcro. The air 240 isn't a case for putting in the max. hdd that it holds.