what size of beam should I use to replace 3 2x8 that is 16 feet?
That's an extremely long span for 2x8s, even tripled. Do you know whether there was ever an intermediate post? Most would suggest that you have a engineer look over your situation. Existing framing isn't always a good indicator of what to use. 

At any rate, a pair of 1-3/4" microlam (or laminated veneer lumber--LVL) beams will almost certainly suffice. The only question is what the height needs to be for your load.