Yeah, the Reed children were definitely in the back of my mind along those lines. But Beric did show up in a single scene of Season 1, albeit played by a different actor. I can't say that I recall him showing up anywhere else prior to the events of season 3.
As for Strong Belwas? I can understand why he wasn't introduced in season 3, since the premise for his introduction was the inclusion of his squire, Arstan Whitebeard. The show didn't go with that, though, and straight-away we knew that the old, bearded mystery man in Astapor was Barristan Selmy. Simply because the television medium makes the concept of a disguise tougher to pull off.