When should I get my next ultrasound?

I got one at 14weeks at the emergency room, 19weeks for anatomy and gender. . 22weeks for a followup anatomy scan. Now Iam 25weeks and wondering when should I get the next ultrasound to make sure my baby's head is down. I also wanna try to get a 3-D , 4-D . . And please Dnt say I don't need anymore because I plan on getting atleast 2 more. . So what's the best time?