3 cm is measuring 3 weeks, 4 is four weeks and so on. It's how they measure ur uterus one cm for one week. When I was 33 weeks I was measuring 35 in the end I was overdue by a week and my little boy weighed 6lb 10 it turns out it was mostly water so as long as ur measuring within 3 weeks of ur due date u should be fine. U might have a smaller weighing baby or less waters or ur baby might be laying in ur back or awaiting a growth spurt but its nothing to worry about.
As long as its no more than 3cm it should be fine. I always measured 2 weeks further along than I was and it was all good.