I am sorry to hear that you are going through this. First, there is no legal separation in the state of Florida. You would have to file a divorce proceeding. Unfortunately, in a divorce proceeding the courts will award time sharing for the children that were born during the course of the marriage. Unless the other children were adopted, the courts will not award time sharing for step-children. However, in many cases the parties agree to keep the children together during time sharing. With regards to whether or not the parent with the 2 children from the previous marriage will be more likely to have custody with the third child, please remember that there are a number of factors consider in determining parental responsibility and time sharing. Moreover, they will make a determination as to what is in the best interest of the child.