Custom providers for Entlib 5 aren't compatible with custom providers for previous versions, so your "compatible with 3.0 up" isn't possible.
For Entlib 5, check out the Enterprise Library Extensibility labs - the second exercise is on building a custom trace listener.
For previous versions no documentation on extensions were ever published, although there are a bunch of examples in entlib contrib.
A third option would be to write a custom .NET Trace Listener class. You'd lose configurability via the Entlib tooling, but you'd gain that cross compatibility (as well as letting you use it outside of Entlib applications).