I have a 40 gallon Hex Tall with one Angel, 6 Lemon Tetras, 6 green cories and real soon a BN. When I first got my Angel a little over a year ago (got him at 1.5") he did the same thing you are talking about. When I introduced him to the tank he went right back to the corner and would not come out for food, etc. I was nervous too, but within maybe 4 or 5 days, he became more playful, learned that I was the one bringing food, and soon started to become best buds with me. A year later he is now 5" or more and has tons of personality. It sounds like this third Angel of yours is doing the same thing, so hopefully he will be fine.