Thanks for your response. This definitely seems like the right things to do. After all, I did commit to A. However, my immediate supervisor would be the hiring

Thanks for your response. This definitely seems like the right things to do. After all, I did commit to A. However, my immediate supervisor would be the hiring manager, and we had good chemistry. I can't speak to how well I get along with company A because I never met any of them, just a quick phone interview, after which they offered me a job.