Hi thingummyjig. The very first question I ever asked on Answerbank was very similar to this so you're not alone.
When my daughter was three she started not only having accidents but being moany, easy to cry, clingy etc. I'd love to give you a cut and dry solution but in our case she went to bed one night, slept straight through for 16 hours and woke up in the morning her old self again with no more accidents from then on in!
Has your son just started playgroup? If so , the new surroundings may be enough to knock him out of his routine. As you said, getting engrossed with toys is very common. Ask him every now and again if he needs to go, even along the lines of ' remember to tell mummy if you need the toilet' may be enough to make him realise that he does need to go. Ask the playleaders to do the same.
The best advice I can give you is the old parenting cliche - it's just a phase. I know it can be incredibly frustrating ( my girl was getting close to being left in the woods to be raised by squirrels) but it might end as suddenly as it begins - and then something else will take it's place! : )