I'm not a child psychologist, but I've also observed this as common behavior, and my intuition is that it comes from learning the numbers as a sequence, not as having actual intrinsic meaning. I've been trying to get my kids to remember the sequence of stops on the subway line we live on for years, and there's a couple they almost always skip — mostly because we rarely stop there. (The one with the ice cream shop, they remember....)
So, first, I agree with the others that it's nothing to worry about. And, second, I'd concentrate on developing number sense — the intuitive knowledge of the real-world meaning of the terms. Don't just count, count actual things. Work on basic addition and subtraction problems (crucially, using counting to get from one to the other, rather than memorizing as "math facts"). That's advanced for a three-year old, so don't expect her to become a wiz at arithmetic instantly, but make a game of it. There are plenty of opportunities in a three-year-old's world for this.
The child development tracker site run by PBS has this note on three-year-olds and counting:
While some children are still learning how to verbally count by ones in the correct order up to "three," the average child can count up to "five." Some three-year-olds will also be able to verbally count by ones up to "ten," and possibly beyond, but not necessarily in the correct order. A very few children will be able to use the "teen" counting pattern to count up to "twenty."
and for age four:
While some children at the beginning of this year are still learning how to verbally count by ones in the correct order up to "five," the average child can count up to "ten," and possibly beyond, but not necessarily in the correct order.
So, I'd definitely relax — but I do definitely recommend keeping at real-world math games as the opportunity presents.