As Kamikaz1_k said, Iron Maiden is probably the most well known band like this. Lynyrd Skynyrd also has three guitarists.
Skynyrd had three, most of the time. The Outlaws had three, they were called the "Guitar Army". More recently, the Drive-By Truckers had/have three too. I'll be back if I think of any more.
XBOX LIVE Gamertag: Jazz Funeral
Currently killing with The Nunts crew in Max Payne 3

Quote by Weaponized
The Eagles

some of Brand New's songs have three distinct guitars happening at the same time (Sowing Season is the best example of this). They have a third guitarist live.
Lord Gold feeds from your orifices and he wants to see you sweat.
Lord Gold probes you publicly and makes your pussy wet.
Now say his name.....
Spelringsteen and the e steer band have like 4, but I'm not sure they really count cause the all just play open chords 99 percent of the time.
Quote by sandyman323
Switchfoot did at one point. not sure if they still do.

They still do: Jon Foreman, Jerome Fontamillas, and Drew Shirley.
In the words of Drew Shirley: ... Sorry, I'm Awesome...
(if anyone can tell me what guitar my avatar is using I'll give you a prize! just comment the answer on my page.)
Quote by ChemicalFire
The point of underground bands is their not popular or famous most of the time. Thus there is a good chance they suck.