Hi, Im Jules, a LPC. I am reviewing your question now and will be back ASAP with a thorough reply :) Please bare with me.
I appreciate your honesty about this situation. Im part of a blended family and I do understand where you are coming from about your concerns. I also understand the perspective of your partner and maybe even a little from where the children are coming from. I think that the kids have probably heard really negative things from their mother. It sounds like she may be a bit immature and even intimidated by the role that you are going to have with her kids. That may lead her to having a great deal of insecurity because having another woman in their lives is probably scary. You are dealing with three kids that are likely going to challenge you and press any limits you set because they have a great deal of confusion about how they should interact with you-- if that makes sense. They have "conflicts of loyalty" and your boyfriend has a "conflicting obligations." He may be absolutely perfect in every way, but he may struggle with establishing boundaries with the kids. Often parents involved in a divorce struggle with setting boundaries because there is a sense of "guilt" that the family system changed. I think that speaking to him about how you feel is going to be the best bet. He has to empathize with your role and you have to explain to him that you would like to grow the relationship with everyone, but he has to step in and explain your role to the kids and not allow that sort of behavior. It sounds like you may feel pretty "alone" right now and you should at least be abel to rely on him. What kind of stance has he taken at this point?
I just wanted to take an opportunity and check back in with you to find out how I can better help you :)