The Raiders didn't have a 4th round pick (I think that was given up for Campbell), so it was either a 3rd or 5th. If they waited for the 5th, they might have missed him, so they used the 3rd.
Considering he is saying he'll do whatever it takes to play, including change positions, I don't mind the pick. The guy can play WR right now or bulk up and be a very fast TE (with probably awful blocking, but other TEs in the league are just as bad, without nearly his speed). Him being picked in the 3rd round isn't a terrible pick, especially when you consider Matt Jones went in the 1st round and the two have nearly identical measurements and 40 times. Jones was a bust, but he did set a strange precedent. Plus, Oakland needs help at WR and TE.