Enterprise admin is a group that has admin rights over a forest and
all domains in the forest. Since you have one domain/forest, the only
real use for this is in AD Sites and Services and AD Domains and
Trusts MMCs. Both MMCs would let you make changes that coudl affect
multiple domains if you had them.
Domain Admins are admins of a single domain by default and cannot
admind other domains unless you allow them that right.
Schema Admins is a group used to allow access to the Schema in a
Forest. The Schema controls what objects and attributes can be created
in AD. You only need to be a member of this if you are making changes
to the schema, which does not happen a lot, but would if you where
installing something like Exchange.