3 guitar live arrangement

[CommonCourtesy]

Didn't know where to put this, but here goes anyway.

Recently I've seen a lot of normal 2 guitar bands use an extra guitarist live at shows, and I was wondering what some of their roles may be, e.g this was Sleeping With Sirens' live Reading set:

http://www.bbc.co.uk/iplayer/episode/p04447vp/reading-and-leeds-festival-2016-sleeping-with-sirens

As you can see the guy in the flat cap is the rhythm guitar, usually doing the chords and stuff. The guy on the other side with the PRS in some songs is doing the leads. At the back there's another guy with a telecaster and what I think he's doing is holding some of the low registered chords or at least doubling up as rhythm. The tone doesn't sound overly distorted either. I'm wondering if some of the amps are set on a crunchy low-gain sound to sound thicker together?

Other bands like All Time Low, Foo Fighters, Green Day, and You Me at Six now play with 3 live guitarists.

So for me I think some of the roles the 3rd guitarist might have are:

1. Doubling the main rhythm guitar, using a less gain-y tone
2. Playing the lower voiced chords with the main rhythm playing other shapes
3. Harmonising with the lead guitar

Anyone got any thoughts on this? (sorry if this is posted in the wrong section)

[Dave Sloven]

There are a couple of local metal bands that have three guitarists: Mammoth and Mournful Congregation.

I'm not really sure what the breakdown of roles really is.  In Mammoth all of the guitarists seem to play leads.  In MC one guitarist seems to play a harmony rhythm part and the other two play the other rhythm part or harmony leads against one another at various times.  Both of these are doom bands, so there is a lot of space in the music and the sound doesn't get so muddled up as it might with faster music with three guitarists

Iron Maiden of course have three guitarists these days

[CommonCourtesy]

Yeah I'm more interested about usual 2 guitarist bands that add a live guitarist for gigs, like how the tones are set up. Sum 41 also use 3 live guitars I think, mainly cos Dave left but came back and they can't just ask Tom Thacker to leave.

I would presume the one not doing any leads or chords could fill out the sections by using octaves or inversions?

I couldn't imagine a metalcore/post-hardcore band in drop b to have poly rhythmic contracsting guitar lines lol.