I've never been ashamed of admitting that I am primarily a D&D guy.  I play lots of RPGs, but I don't study them the way I do with my main game.  I own and enjoy the 3rd edition of Ars Magica though.

What is the current state of the license?  Which edition do you enjoy most and why?

If I was going to start a 3rd Ed campaign, what should I know about the other editions?

I'm quite in love with the 5th edition. It has a more readable design than 3rd, it doesn't need to suffer White Wolf's metaplot, and it has a few little bits and tweaks rules wise that make it more usable. Mostly it is simply the little things that it changed (Weapon damage IIRC, study rules) that make it better. Yet we're splitting hairs really.

Since I first started reading and running AM I've found them all very good games. No edition is innately superior, although I prefer the latest version for keeping the simplicity and being both more readable in language and more tightly "designed."  I'd need to re-read both editions to give you more specifics and I sadly don't have 3rd anymore. Making that non-doable at the moment. I'm also not currently running 5th either, although I was thinking of starting a game of it when my current players sort a few things out. I think the troupe style play and the society of the magi and their companions might work quite well.