Thesis supervisor wants me to publish my work in his own conference
I think there would be a conflict of interest, if your co-author is also among the chair people of the conference.

Apart from that, you should not publish anything without your other supervisors acceptance and/or without informing them. That is: do not put their names on something you submit if they don't know or don't agree with being named there, AND (especially relevant in this case, it seems) do not submit something where you omit a contributing author. Author here doesn't need to be someone participating in writing a piece, but already being part of a research project.

Therefore, if you want to publish at this conference (independent of A's role in the conference), then send an email to all your supervisors and ask what they think about publishing/presenting your work there. If they don't know about A's role in said conference, and you worry about it, tell them (maybe omit A in this case, if you prefer) about the position of A in it.