I think you are on the right track. If you want this to work and not just work but work well it may be worth it to open up a hole for exhausting. You could do fresh air intake but if you only plan on making only 1 hole exhausting is a good option. Is smell a concern, if so attach a carbon scrubber to the exhaust. If smell is of no concern since the air will go in the attic and it's only three plants attach the exhaust to an air cooled light that pulls its air from the hood in the room. Cools light and exchanges air in the room but does not take care of smell. Can you get a 250 hps or a 400, I think with a large enough fan, 150 cfm or more, and a vented hood you could turn those plants into three monstrous babes. Good luck let me know if I can help. Currently running a 400 air cooled in a closet with good results thus far.