One guitar, or three?

Eagles, of course. Lynyrd Skynyrd. Crosby, Stills, Nash and Young are other faves that come immediately to mind.

Having said that, I have a little trouble with the notion that “three is more musical than two”, or “two is more musical than one”. Depends on what you do with what you got. One great guitarist is far better than three average guitarists. I can name solo guitarists (never mind one guitar bands) who are far more musical than some bands with two or three guitars; in my book, anyway. Knowing that you are a proponent of the “less is more” kind of playing (me too), I’m a little surprised at this notion.

The Motown house band, The Funk Brothers, had Robert White, Joe Messina and Eddie Willis.

Emmylou Harris Hot Band, actually 3 guitars and a pedal steel.

Check out Wilco, with Nels Cline, doing “Impossible Germany” from this clip: https://youtu.be/7I79m7_kAnA Very tasty, with Cline shredding at one end of the stage while Jeff Tweedy and Pat Sansone are harmonizing at the other end.

@onhwy61: Yeah, many Soul bands---who commonly have great rhythm guitarists, a thing very different from a lead guitarist (John Lennon was a fair rhythm guitarist, Don Everly a great one)---and, especially, Country Bands---who commonly have at least one acoustic rhythm guitarist (in addition to the singer, if he plays) and another on electric, plus an electric lead player or two, and a pedal steel, dobro, mandolin, and fiddle player (no wonder I like good Country as much as I do ;-) never have only one guitarist.

But remember, I specified Rock music bands, where a single guitarist is much more common.

Singer/songwriters often hire as many musicians as they can afford. When I saw Lucinda Williams for the first time after her Car Wheels album sold a million (I had seen her playing around L.A. for years before the success of that album changed her life, one time in a pizza parlor to an audience of a half dozen people), she was playing acoustic rhythm herself, had Jim Lauderdale playing acoustic and singing harmony, two electric guitarists (one of them being Kenny Vaughan, now of course playing Telecaster in Marty Stuarts’ The Fabulous Superlatives, imo currently the best band in the world), plus a pretty expensive drummer, the great Jim Christie, who quit Dwight Yoakam’s band when Lucinda offered him a job. My gawd she and they sounded great that night!

For years Emmylou had the best band in the land---The Hot Band, featuring at various times James Burton, Albert Lee, Ricky Skaggs, Rodney Crowell, Tony Brown, Glen D. Hardin, Hank DeVito, Emory Gordy, and either John Ware or Ronnie Tutt on drums. They weren’t named The Hot Band for no reason! Emmylou, having great taste and surrounding herself with the best musicians in the world, now has Buddy Miller playing electric guitar and singing harmony with her, plus leading her band. He also leads the band at the annual Americana Awards Show. He’s a great record producer too!

While I agree that two is often better than one, for my tastes, a third is usually one too many. The degree of commitment required to listen carefully enough so as to stay out each-other's way is usually more than most guitarists can summon in such a situation. 

Never "grokked" Moby Grape.  

I did hear Emmylou with the Hot Band and they did fine, but you specified Rock, so nope.