I think you got a good deal on some classic Keith Brothers Traveller material. I also think you paid about market value for material of that era and especially since you did get the set (3)
That seems pretty fair to me. I've seen them go for a little less ($5-6 each), but I've also seen them go for a lot more ($20+ each). But most importantly these are three really great adventures, and if you've never read/played them before you and your players are in for an enormous treat!