Jorge, we have tested primarily with Eclipse 4.2 but there shouldn't be any problems with Eclipse 3.8.
I assume the reason you don't want to use Eclipse 4.2 is because you've seen some of the performance problems Eclipse 4.2 has on some systems/usecases ?
If you see a problem installing jboss tools on eclipse 3.8 please let us know on https://jira.jboss.org/jira/browse/JBIDE and we'll look and see if we can make it continue to work on 3.8.