I have 3 DC's, with the original as GC. When I configured the new DC for Exchange, i made it a GC as well. best Practices suggest it not be a GC. How negatively can this impact my server? I know it's usually best to heed their advice, Im just curious as i now have a good config, and afraid to change anything...
Best practice is to always separate your Domain Controllers from ALL other applications. I would suggest demoting the DC that you have Exchange installed on and making the additional two DC GL servers as well.
If your on different subnets and the Exchange is on the only DC for that network, I would propose creating a new DC. If its a small network it doesn't need to be super powerful, if your on 2008/R2 you can even look at doing an RDOC (Remote Domain Controller) with a minimal install.