The easiest way to determine a pregnant rabbit is to palpate...
1) Place the rabbit on a flat surface facing toward you.
2) With your left hand, hold the rabbit firmly (I hold the ears and scruff of the shoulders in a firm, but gentle fist)
3) With your right hand, reach up under her belly from the side. Make a"cup" with your fingers upwards (thumb on one side of rabbit, 4 fingerson the other side of rabbit, belly resting in the middle, palm of yourhand).
4) With your fingers and thumb, gently, but Firmly squeeze her bellyfrom just above the genitals right on up to just under her rib cage.Don't be afraid to really dig in there, sometimes it's really hard tofeel them in there. You will be feeling for any large round "balls".Easiest description of the feel of fetus in the womb is a grape. Feelsfirm, but slightly squishy... and depending on how far along she is, itcould even be that size.
Some rabbits will tense up and try to get away at first, but be patientand persistant... One way or another, you need to find out if there'sanything in there.
Now, since you said she has been with a buck about 3 weeksago, full term fetus's are larger and you will be able todefinately feel a head... Think of a Large Jawbreaker.
Hope this helps, Good Luck!