Assuming everything Bob told you is correct, Andy and Charlie want to fire you and don't want to discuss it with you. Bob at least wants to have a conversation. So I would go back to him in person or send an email and say something like:
I was so surprised by what you told me that I needed some time to process it. I had no idea my performance was not good enough for Andy and Charlie and that the comments from Andy recently have been related to objective performance issues. If you have 30 minutes, I'd be very grateful for some more specific direction about ways I can improve.
For an email, finish with an expression of how much you have enjoyed working there, want to continue working there, and want to be the very best you can be, along with some gratitude that he even told you anything about what's going on as far as the perception of your performance is concerned. If you're going to do this in person, practice the sentences a little so you get them reasonably smooth.
If Bob doesn't have time to help you or replies that he's not really your manager, you will have to approach either Andy (who has expressed a problem with you, but not in a way you understood) or Charlie and ask them what you need to do to get your performance where they want it. You should not tell either one that Bob filled you in.
To Andy I would avoid email, and find a moment without no-one else around and probably say:
If I were getting my performance review today, it wouldn't go well, would it? [pause. Nervous laugh especially if he laughs.] I would really appreciate some specific direction about how I can meet your expectations in this role.
(I know, you asked if anything was wrong, but that's nowhere near as specific as Am I doing a good job?) You probably won't get the direction there and then, but if you're lucky you'll get an appointment for 30 or so minutes later the same week in which he will tell you everything that's not good enough.
To Charlie, perhaps:
I think Andy is not happy with my performance. Is there anything you can tell me about how I'm doing?
I would be vaguest with Charlie because you haven't reported anything about what he has said or done. Chances are he will send you to Andy (because he has the problem) or Bob (if he's actually your boss.) So tempting as it may be, don't start with the most senioe person. Start with the ones who have told you your performance is not what it should be.
Whoever you talk to, whoever agrees to talk to you, ask them to please help you to do better. And listen carefully to what they tell you. They probably won't tell you again. While you may be scared to hear a big list of complaints, not hearing it won't keep them from acting on it. Your only chance to turn this around is to know what they care about and what they think is a problem.
On the matter of whether they have messed up, should have said something, or are handling this wrong: they almost certainly have and are, but that does not matter. I remember dealing with an employee who was not meeting our needs, missing deadlines, not communicating, and upsetting other employees. He wanted to have a meeting about how I was not a very good boss. I told him "I may or may not be a good boss, but I am the boss you have. And since I own this company, if you need a different boss you know what you will have to do to get one, right?" You need to work out what they want - if you have to interview it out of them, if it is all unfair and wrong, well it is what it is. You can get new bosses by leaving, or you can try to work out how to work for these. But I promise you that if you try to show them what horrible bosses they are it will not help your cause at all. It may not be your fault that you let them down, but it sure is your problem.