My band consists of : 3 guitarists, A singer, a drummer and a bassist
I would like to know if you know any Classic Rock, Metal or w.e. songs are good to play that would accompany all three guitar parts.
Wow, three guitarists, that's cool.

Hmmm, the neat thing is, you can look for songs that do overdubbing in the studios and actually play it live.

I would suggest some SoaD from Hypnotize or Mezmorize. Alot of that (Soldier Side - Full song) and whatnot have many guitar parts.

Or you could take a song and even add your own improvisations into it.
Iron Maiden now have three guitarists.

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you would be able to do the harmonies
Something by Avenged Sevenfold, probably. They usually have harmonies between 2 guitars on the leads and a rhythm part in the backround.
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Maylene and the Sons of Disaster have 3 guitarists and all of their songs are super easy.
Chili Peppers - Dosed.

Put your three guitars to use and if one of you is really good, you can solo over some of the parts as well.

Edit: Plus, it's one of my favourite songs ever so I just wanna spread the word