It's a good game, my personal favorite in the Forza series, but Forza 4 is much superior, if you don't already own it.
Whoa wait a minute! Forza 3 is a completely different game then Forza Horizon 3. (Turn 10/ Microsoft runs two Forza Series', one is Regular Forza and then there's the open world version called Forza Horizon.) As for my opinion on the games? Well I played both Forza Horizon 1 & 2 but not 3, but from past experience I say Forza Horizon 3 is worth the buy. Forza 3 on the other hand I can't say, as I have never played it.
It's a good game when it comes to content, but the map can get boring very quickly. DLC's are very expensive imo too..
I love it, I wish the map was more varied though, almost all the roads are either super wide and fast or dirt roads.