I think it would be fine to jump in whenever. This is a pretty deep game and starting in to learn the basics before 3.0 seems like a solid idea, that way you don't feel quite as lost when there is even more new stuff to learn with the expansion. I think 3.0 is more than a month away at this point so plenty of time to explore the foundation systems and figure out what kind of skills and playstyle you find most enjoyable.
I wouldn't focus so much on the builds as how the skills feel, learning how currency works, vendor recipes, the mapping system, exploring the passive tree, etc. In Path of Exile pretty much all of the game systems collide with one another (intentionally), so any gaps in your knowledge about one of those systems has knock-on effects of the value you can extract from the others. It's a very knowledge-based game so the more you learn about it the more you'll get out of it, and most of what you arm yourself with in 2.6 will be wholly relevant in 3.0.
