Not sure what you mean by "upgrade path", but logistically 3.x does require paying for new license(s). FWIW, I migrated directly from 2.6 to 3.1, bypassing 3.0 and 3.0.1.
The documentation you're looking for is probably something along the lines of "Data Migration Guide for LAN Management Solution 3.1", which lists topics such as "LMS 2.6 or LMS 2.6 Service Pack (SP1) to LMS 3.1":
We are pleased to announce availability of Beta software for 16.6.3. 16.6.3 will be the second rebuild on the 16.6 release train targeted towards Catalyst 9500/9400/9300/3850/3650 switching platforms. We are looking for early feedback from custome...