my piece of opinion

Is agent B the seller's agent? To me, it is impossible that he/she could represent you without a buyer agent agreement. Typically, the whole commission is 5% - 6% of house value, and the buyer agent and the seller agent will get half each.

You are legally bound to agent A. Sounds to me that he/she has some ethical issue. If you already signed a 3-month agreement with that agent and the agreement has not expired yet, why he/she asked you to sign another one, worse worse, no expiration date?

If I were you, I would try to understand both buyer agreements you signed to see which one is a valid one. I am more concerned about the second one. Try to consult your local realtor organization and get some legal advice.