Which is better- one 5 channel amp or 3 stereo?

Synergy aside;It depends on what amps,and what speakers.---Not to mention how much one wanted to spend.A 10k amp will be 'under-performing' driving 500 dollar speakers.---No matter what processor one is using.

I am using three B&K ST-140's.They don't put out a ton of power but they sound fantastic.One thing that helps them is crossing everything over at 80 HZ.This way they just buzz along with no bass worries.I tried a bunch of multi ch. amps under 1000.00 and never heard what I hear out of the B&K's.There are draw backs to this in HT because the amps have no triggered power ups.I had to make my own delayed 110vac switch box to fire up all my amps.Now the system is one button "ON". What I would really like is to get three Amber 70 wpc amps and start another system in a spare room.Those amps are old but they sound so.. sweet.

There is another thought, you could have a main stereo amp/two mono's and a five channel. The five channel is used for the center (bi-amp)and surrounds. This would set you up for a six speaker system. When you are listening to 2 channel you would have the main amp(s) to power the main speakers. As Avguygeorge said, How much do you want to spend?

I recently made the transition from 3 to one multi -- I had a krell for the fronts -- plenty of slam and punch and resolution, a Classe stereo on high and low of the center -- very delicate and detailed, crisp and clear resolution of dialogue, and a Legacy for the rears, with fantastic punch and resolution. I came to those matchings after moving each of them all around for the best position. This was alot of fun and alot of work, but finally I tried out a multi (Cal Audio, 500 watts per channel) -- the first time I heard it, I realized how much my system could have benefited from the cohesion and coherence that comes with a multi. There are drawbacks however -- my speakers are large and I listen loud, so the amp tends to draw alot of current from a single outlet all at once on peaks and this has proved too much at times, but the system came together in a way it had not before. However, if you use 3 amps from the same manufacturer for all 5 channels........