Honestly I don't think you need more than 2. I have two on my 75 with a powerhead set to come on for 5 minutes every hour. The powerhead stirs up the water and makes sure "stuff" makes it to the filters.
I don't think it'd quite fit. Wouldn't removing the center brace be scary on a 75? I'd think failure would be eminent. Especially with the weight of all those filters full of water hanging on it.
Yeah, I am with the AC101's all the way, I have two pumping a 125g along with a DIY wet/dry..............The AC's are simpler and have less moving components...............
You can always put the 3rd filter in the front, but some may think that would be ugly or in the way. 2 emp. 400s should be fine for a 75 if it is lightly stocked. You could add a #5 sponge filter for more bio
I'm pretty sure the center brace is structural on a standard 75 gallon tank; remove at your own risk. You can put one filter on the end but you will need to modify the lids.