There is no answer to your question based on what you've said. That is, we can speculate a lot and keep it up for weeks but with no real certainty. For instance, you may have lost the other birds because of a hawk attack
that got one, scared one other to the point he flew beyond his area of recognition and got lost, and the one that returned is both traumatized from the violence of the attack and misses his fellows terribly to the point of depression.
OR, they were all coming down with an illness together and the other two didn't make it back because they might not be as strong as the one that you still have. So, you see there's just no definite answer. If it were the first scenario, then hopefully one or both of the missing two will eventually show up. That has happened to other members here to the point of it having been literally months since the initial disappearance. You just never know.