The first Fable is a fairly decent lighthearted action-RPG. It has some charm and interesting world simulation elements few other games do. On the other hand its combat and enemy diversity are kinda poor, it has really clunky menus/inventory/etc., and its world is very limited and linear, with little to no real exploration other than a few side paths. I enjoyed it at the time but I don't know if it's aged so well.
Fable 3 is better in some ways, mostly in offering a more wide-open world with more fun exploration. I also enjoyed several of the side-quests and the game still has much of the Fable charm intact. But, the game mechanics have been dramatically simplified to the point where calling it an RPG is extremely questionable, there's basically no challenge at all, and it has one of the most infuriating menus ever. Heck, it's even a pretty lengthy game compared to the first one, probably 30-40 hours to do everything.
Never played Fable 2 but some seem to think that, as far as gameplay goes, it's the best of the bunch. Overall though I have no strong love for the series, it's not bad at what it does, but what it does may not appeal to you if you want something deeper or more serious.