If you can prove that you have been a stay at home mum for 3 years, ie have somebody who will verify the exact dates, a friend maybe then it shouldn't be a problem. I had a situation before joining my previous employer where I had been out of work and not claiming benefit for over 6 months due to having surgery, a simple letter signed by a friend coupled with criminal record checks etc and it's no problem. The best thing to do is gather as much info as you can, previous P45 dates etc will help, just be sure to fill the gaps.