I first felt my movements before 20 weeks, and by 20 weeks you could definately feel them from the outside, but everyone has commented on how my one month old daughter is unbelievably strong. Some days though she did not kick much at all, and i was told that it means they are facing your back so you cannot feel them, which is more than likely what is happening to you.
A midwife when i was in labour, asked me what i thought i was having, and i said a boy because my baby was soo strong and active, and she guessed it would be a girl because from her experiences, boys are less active, so you just might be getting the little boy you want soo bad... I hope
Plus, you commented that you had two girls previously who started kicking earlier, so the midwifes philosophy, may just be right!
Whatever the outcome, i hope you have a healthy and happy baby.
If you are concerned about the lack of movement, just ask the doctor, they can tell you from the ultrasound if all is okie dokie!
Oh, and remember, even if they say you are having a girl, it could be a boy who is hiding his little thingy! So, don't give up hope!!!!!
Good luck, and make sure you write and tell us what you are having