Snow suits for may babies

Hi ladies ..I am pregnant with b/g twins and bought some matching snowsuits for them in size 0-3. My original due date is 28th may but doctor said i will not go past 37 weeks and they will induce me at 36 weeks if the babies dont decide to come any earlier(finger crossed). So the question is should i keep the snow suits or change the size for them to 3-6 months as it will be august when they will be 3 months or should i change it to 6 to 9 . Thankyou. I am a first time mom and struggling to see how much and what sizes i will need according to the weather. i am based in manchester if that helps xx