*** Moved from K1 Progress Reports to K1 Process & Procedures ***
Just need one I-129F (and one filing fee) for the mother, listing her children in the appropriate section of the petition. DHS does not need to know that you want all three to come over at the same time, but DOS will need to know. So after DHS approves the I-129F and sends it to the consulate, your fiancee and her kids will need to submit a DS-160 form for each, and pay the MRV fee for each as well.