when you're talking about the one-guitar thing, there are tons of ways to go at it. songwriting style shouldn't matter, but keep in mind that some people write better for different size ensembles, you know what i mean? listen to alot of hendrix and take note on how he fills that void where the other guitar would be. hendrix plays his solos behind minimalistic drum and bass, and it sounds wonderful. you will have to understand that the less instrumentation going on at a particular time, the more you 3 will have to step it up and fill that void with musicality. less musicians in a band means more weight on the shoulders of the few.

as far as writing your songs, it would benefit you to be more "riff-based". basing your chorus around a catchy guitar part can more than make up for the lack of band members. remember to think like a minimalist, because that's what you're working with. don't try to go too out of your element (and your playing ability) because you will find out the hard way that you will hate to play shows and you will hate to write songs if you are expecting to make rock operas with 3 people.

i'm assuming the bassist or you will be putting down on vocals, so your songs must be more rehearsed than you might have had to be otherwise. improvisation is fine, but don't be expecting to belt out your songs through a mic and think you can just "go by the feel" on the guitar. you have to have your syncopated rythyms down 10x better than you would have otherwise.

i know these facts are a bit scatterbrained, but hope they help!