This isn't grammatical in any case. You need "with" (or "of" or "at") before the name of the organisation.X is a partner ABC Ltd.
No, they are just partners. You cannot be a partner on your own.I think A, B, and C are partners to each other.
If there is a context in which you specifically need to say that A is a partner in relation to B, "of" works best.I think A, B, and C are partners to each other. In other words, each one of them is a partner of the others--A is B('s) and C's partner; B is A('s) and C's partner;