I'm really sorry to hear of your loss. Sebastian is a lovely name. I hope his brother and sister are doing well at home with you.
We also lost a triplet boy, Ted, who is Laurie's identical brother. My little boy and little girl are both just over 2 and a half. We've only got as far as saying goodnight to Ted each night. They are just starting to understand that they are brother and sister (and calling each other brother and sister) so over the next few months we might start introducing that Ted was/is their brother. We're just feeling our way really, but like you want to make sure he is part of their lives without casting a shadow. You summed it up really well and I am sure the right thing is to try to do things as naturally as possible. I'll let you know how we get on.
Again, so sorry that you lost your little boy. Please feel free to PM me if you'd like to.