The Revival Catalyst will be available from day 1 in Season 3.
Blizzard outlined changes planned for the Revival Catalyst and charges in a Forbes interview.
They will enable the Catalyst immediately when Season 3 begins, but you are going to earn 1 charge every 2 weeks instead of every one.
Scarborough: We’re going to be moving to a system where we unlock the Catalyst at week one. It is available, but you are going to earn charges more slowly, so every two weeks instead of every one. There are a few other tweaking and moving of numbers to make that make sense.
What this effectively means is that if you are a non-raider, you are not going to be nearly as disadvantaged as you’ve been before. You’re going to be starting to get pieces from the jump, though you will have to wait two weeks for your next one, and two weeks for your next one.
The point at which most people would get their four-piece [bonus] does not change, but the point at which you can start collecting them, especially if you’re a non-raider, starts sooner.
You can read the full interview here.