Why is your 3 year old dog suddenly pooping in the house?

There are many possible reasons. Firstly, make sure any change in your life pattern is not affecting the dog. It could be the dog's way of grabbing attention, if the dog is not happy with something, he/she wants you to take notice.

Another reason could be the dog's dietary needs have changed, either you are feeding the dog too much or too little protein. Try rotating your dog's food.

I am assuming your dog has no other health related issues.