Hi M.,
What a great letter. It addressed very common worries. First, think of the places you go with your son. Does he take off his clothes and pee outside at all those places, or has he figured out there are places that he shouldn't do this? Either way, don't worry. Arrange an appointment with his teacher prior to beginning preschool, and mention these behaviors. The teacher will have experience with home behaviors and be able to head them off.
Why does he think you are going with him? Have you told him that you are so happy to be bringing him to a place where he will make friends, and that he will be safe with his teacher, and that you will go home when he is in school so that you can clean the house? These messages let him know that you are certain of what you are having him do, and where you will be. That helps with the security issues. If you have told him these things, there's no need to repeat. He is saying you will stay with him because that makes him feel secure. Again, make a plan with the teacher. If you will have the twins with you, it probably would be best if you hugged him once, kissed him twice, said good-by and left. If you will not have the twins, you have other options. Ask the teacher to plan with you what would be best for your son. If you are giving him a pep talk, I would avoid using the "You're a big boy now." because he may very well not want to be a big boy, knowing his Mom is going to be home with the twins. Sometimes it does help to say "You're 3 now. Three year olds go to preschool."
Have a wonderful fall.