السلام عليكم I know of 3 conditions of repentance .Stop the sin, regret doing it, resolve to never do it again. But I’ve heard another condition, one must believe that his repentance has been accepted. Is this condition valid & what is the ruling on a repentance where ones unsure if it’s accepted.

What you were referring to is decisiveness, not how you worded it. There are others as well: his repentance is before the sun rises from the west, and it is not at the moment of death. Anyway, all in all, I believe you know exactly what repentance is, and I don't want you to fall into excessive questions and digging too deep.