Well, I'm always a bit leery when someone says a 3 yr. Old is "dead quiet." It's like they aren't even old enough to have an opinion! I always suspect that the horse is quiet because no one has required much of it yet. But, I know the rare horse does come along that truly fits that description.
If the horse passes a vet check, and if someone objective who knows both horses very well and you very well approves, and if you have the money to invest in having someone else put some time in with this horse... Then I hope it is a great one for you.