There are different ways to get into poly. As a young single person, as a choice for all future relationships. As part of an established couple who both feel something is missing, and choose poly rationally and with love. And then, as a couple wherein one (or both) partners have cheated, are discovered or come clean.
The third option is, of course, the most painful, because the cheating has caused a breech of trust. So, suddenly, the cheated on partner has to deal with feeling humiliated and betrayed, and then have to force him or herself into bringing the secret lover into his/her life. Not an easy thing to do, and imo, your h and his gf should not expect you to be on board after just a few short weeks. They should be made aware this process can take years, and much work.