I totally know what you mean man. I've quit some many bands because I was not satisfied with what was going on musically. And when I tried to bring something new into the mix, I got to hear the same words. I was younger at the time and now I have learned somethings that might help. I'm not taking anyone's side despite what I will write, just trying to help and hopefully present something that you and your friends can see as common ground.

About the lyrics won't fit the rhythm, there is some substance to this. When the vocals enter, the music should back the vocals, not the other way around. You don't wanna do anything extra here that will pull any attention away from the vocals, rather you should find a rhythm that will support it. I understand what you mean by the rhythm always being the same, which is usually caused by the singer using his guitar to kept time. Using by strumming 8th notes alot, right? A good way to remedy this is by first singing the vocal line accapella (without any music, like 4 black guys on a street corner and philly). Then once you get a good rhythm and melody for the vocal line, have the lead singer keep singing it while you come up with something on the guitar. Remember your backing it up, restrain from doing anything that might pull attention away from the vocals. There are always vocal breaks where you can add your thing on the guitar. Trust me if you approach it like this your songs will be alot more structured, and better. You'll probably get to add the rhythms you want and you guy's should be happy on both ends. It's worked great for me.

About using 3 chords, use that as the structure of the song. Some of the greatest songs were written with 3 chords and it's where all great songwritters start. It's how you use them that makes them great songs though. Even the chord progression G - C- D has infinite possibilities and you can create something new with it (like you want). Your friends sounds like a songwritter first. Most songwritters will just play the same rhythm using common chord progressions because there goal is to write a good song not a musically extravagant song. Nothing wrong with that either. It's just you guys need to understand the job each one of you guys are gonna perform in the band, and work together to make the big picture. I'm guessing your gonna be the one to make the music great, so my advice is to take what the other guy does (like g-c-d c.p) and make the music alittle better using the structure he gives you. I hope you understand.

Another thing, since you meantioned your more of a metal dewd and your friends are more or less into punk. Check out the band "broken bones". These guys were together before slayer and all those other thrash bands, and there music is pretty much thrash punk. It's not the greatest music but it might help you get a picture on how punk and metal can work together.