Which parent has custody rights when there is no court order?

My wife and I are splitting up and we have three children. She says that when I move out of the house, the children are staying with her and she’ll decide when I get to see them. I have been advised that I can’t go to court to file for a divorce or for custody until she and I are no longer living together. I want to know if that is true. If it is, who has custody rights to our children until we are able to get into court and have a judge decide? Can she really keep me from them if there is no custody order in place? Am I within my rights to just go and get them from the house after I leave?