K.S.
This is happening because the 3 year old is feeling neglected. This is so unbelievably common. Anytime a new baby comes home, the older sibling is no longer the center of attention. What is the absolute one thing a small child can do to get a little bit of undivided attention? Easy. She just needs to stop using the toilet. Then she gets all of that one on one attention to get her diaper changed.
When my twins were born and BEFORE my son had a chance to regress in the potty training area, I used to change the twins' diapers with a big and sad sigh and would say things like "Babies, I wish you were a big boy like your big brother Alek and could go in the potty all by yourself." Or, when one of the twins would start fussing I would say things like "Baby, I know you want Mama to come and change your diaper, but I'm a little busy reading your big brother a book (or whatever else I was doing) so I need you to wait a couple of minutes."
My oldest really enjoyed the positive attention, and every time I would change a diaper during those early months he would tell the babies that they could be big boys and learn to pee in the potty like him.