@ArpanaR
Honestly, this is the best possible foundation for a actually stable polygamous relationship. They usually fail because either the 'first' or 'second' partner doesn't actually like the other or become unsatisfied with how much love they get, and become jealous and bitter.
In this case however, the relationships are not only stable, but they all actually love each other (although in different ways).
Maria romantically loves the MC and the Prince (although the MC comes first)
The MC loves Maria both romantically and as a friend, while she only loves the prince as a close friend.
The prince loves Maria romantically while he only loves the MC as a close friend.
In this case, it can work out as a relationship like a H2O molecule, with Maria in the center receiving romantic love from the both of them (and probably a lot of sex, lol) and MC and prince as the side, where they mostly have mutual respect and friendship.
Honestly, if there is ever any complications, it wouldn't be between Maria and MC (since Maria and MC love each other and MC doesn't seem to even want the Prince's romantic love), It would be between the Prince and MC on who gets loved more by Maria.