Children also have "stress", just like adults and it gets manifested in different ways.
Regression, is a symptom of stress.
Do not do anything that will make it worse. Being an "eldest" child, all of a sudden, is a great stress for some children... it's a BIG thing to weigh on their shoulders.
Think about that. Do not expect him to be older than he is, do not put the responsibility on him to be "perfect" and to be "an example for his younger siblings." They are simply not mature enough for this expectation, nor do they want this responsibility. It can be much to much.
My daughter, when I had my son, would actually tell me things like "Mommy, I'm just a little girl... don't expect me to be grown up..." or, "...don't expect me to be perfect..." And what a jolt to reality that was... and boy did she teach me something. Yes, my little girl was right and wise. When I really key into my daughter amidst the "busy-ness", I can really tell when she is feeling "stressed" and perhaps I am expecting too much at a certain time from her... then I back off. I talk with her, I even will 'apologize' if need be. I think, its very important for a child to HEAR their Parents apologize to THEM too. I teaches them about life and about relations, about family.
It is not just about spending time with the eldest child... it is about what the Parent(s) expect of him, how he is viewed, how he is nurtured, how he is integrated into the "new" family structure, how he is enabled to "be" and not expected to be, etc.
I once had a child vent to me... she was about 6 or 7, and she sat down by me and just started spilling her guts... telling me how "miserable" she was and how her parents ALWAYS blame her for anything and for any cry her younger siblings make, and how she always had to do everything, and how NO ONE listened to her, and how she always had to "baby-sit" her younger siblings... I told her she should talk with her parents about it... and she said no, because her parents never listen to her anyway, that they are too busy, and they never believe her anyway. How sad it was to hear this from a young child. BUT, this is the way they feel and see from their eyes. This child had lost all hope and belief in her Parents that they would trust her or be there for her. All her parents did was "expect" things from her since she was the oldest.
Anyway, your son will be fine. It is a phase. When I had my 2nd child, my daughter had a few pee accidents. It's OKAY. We talked with her, thoroughly. We reassured her, we just loved her and did not make her feel "odd" about it. We also gave the head's up to her Preschool Teacher. It's totally NORMAL. But, a child who is going through this, needs our understanding. Your son is only 3 years old.... so young still. Toileting 'accidents' WILL happen still. Even at 4 or 5 years old it may happen... and then there will be night time pee accidents too. It's okay.
Take care,
Susan