problem with having 3 ac's is the 120 gallon is only 4 feet long. so the ac's are what a foot long a piece? that takes up three feet of room in the back and with the frame and everything might not even fit. i only have two filters on my 55 gallon an ac 50 and a whisper 40. but i only have 6 danios in there. i will get more filtration soon but when i do i will have to remove one of the filters and replace it because i have a split frame and plastic hoods. so i would suggest two good H.O.B. filters that each get at least 350 gph. (not sure which brand you like) and then you get a good canister so you have more room in the back and you have great mechanical and biological filtrationI would go w/ two ac 110's for smaller fish/less crowded conditions. If you have more/bigger fish, then I would go w/ 3 ac 110's. I have 2 of them on my 125, and they are great. I had a fluval 404, but it crapped out on me (several internal pieces have broken)
