If anyone, literally anyone, gives you ultimatum of marrying within 3 months (or 4) at any point in time, DENY IT!!
It’s just a trap. Reasons may vary for the ultimatum, your relationship dynamic may be any, in fact, you may also want to marry that one.
May be it’s not good for you or you both, and under the pressure of such an ultimatum, you may get married and thus settle for something which was not right for you.
The whole idea and vibe of an Ultimatum is the problem here. What’s good for you is never forced in any way whatsoever.
Love, emotions, feelings, affection etc. on one side, this kind of mental pressure is another level. Better leave than regret.
(I used 3 or 4 months because it’s such a short time to think through things about a decision of a lifetime.)