Losing 3 inches off bust?

Hello. Im new. :3
Im 18, 190, 5'3.
Bust - 43", waist - 36", hips - 43"

As far as how I look, Im relatively okay with it.
I do have problem areas though.
My arms have always been my worst body part and now after gaining 15 lbs my tummy is bothering me too.

Along with losing weight Im hoping to reduce my bust.
Theres this cute dress Id love to have for prom but the largest size says I need to have a bust of 40.5 inches.. Im 43 :[.
I told my mom Id lose the weight so we should buy it now before its gone but she doesnt want to.
Im terrified that when I do go and buy it Itll be too late. T^T

Do you think its possible losing 3 inches from my bust in time for prom?
Also about how much weight do you think thatll be? I know its near impossible to tell but I want something to go by.