Could they continue? Yes, of course they could.
Would they continue? I very much doubt it.
Should they continue? I don't think so. Obviously, losing Glenn is a huge hit for them. Losing Glenn plus Don or Joe would make it just about futile to continue, IMHO. They would have very few of the original songs with the original artists. I love Timothy but I would consider him more replaceable. Without Don or Joe, it wouldn't be impossible but I don't think it would be right, and I doubt very much they could make it a commercial success.
If the question was should they continue without Don or Joe, if they still had Glenn, it would be much more difficult to answer. I'm not sure on that one. It would be very tough, no question, but then it's very tough without Glenn.