Unless you signed a contract from company B I wouldn't reject the other offers just yet. Instead I would consider asking some more questions at companies A C to learn more about the role, but ultimately, to give you some extra time until you secure company B.
You can only keep this up for so long, so if you still haven't signed with B then you need to make a decision that you're going to stick holding out with B, or consider actually accepting A or C.
If you don't care about A C, then simply tell them that you have already accepted another offer that you just couldn't refuse and thank them for considering you. You shouldn't look bad in their eyes, and this gives you the opportunity to reach out to them in the future.
Otherwise if you don't mind jumping ship to A C, you can accept one of them, or play it risky by giving them a counter-offer while you wait for B. Trying for a counter-offer may fail spectacularly though and you run the chance of losing out in all companies!
Congrats on the offers, and best of luck.