You sure can.
I have a pre-made Haier keggerator unit that will fit 3 cornies. The tower that came with it is 2.5 inches as opposed to the aftermarket 3 or 4 inch towers available. I was going to invest in a 3-tap 3 inch tower, but I did not want to part with that much money for one, so I decided to try and modify mine first.
It was a challenge to squeeze 3 taps and lines into a 2.5 inch tower, but I got it done just fine. It works great and saved me some money.
The standard tower material is very thin, so when you drill it, be very careful you dont crush the tube, or crack it if its made of plastic. I wedged a piece of wood in it to support it during drilling.
Here is a picture of it with the first pour after I modified it. Sorry, I dont have any in process pictures.