Well, it says in the pattern that you can do the math and calculate the dimensions of the wearer to the glove. Since a 3 year old's arm is slightly different than that of an adult woman's arm, I would do some more math and plug that in somehow. I am sure if you just measure your daughter's arm, or a simliar sized girl's arm if its going to be a surprise gift for her, and plug the dimensions/measurements into the equations provided, you'll get the measurements you'll need.
Unfortunately, I am not very good at any math. I barely got through basic Algebra in one piece!! Maybe someone else, who is better at that sort of thing, would be so kind as to take pencil to paper and figure out the calculations?
FYI, I would make them to fit a slight bit loosely, and out of a yarn that has some elasticity to it, so that your daughter can wear them as she grows a little.
My knitting blog: Yarnolicious
Oceanblue28 on Ravelry
Garter Stitch Bag in variegated shades of pink and purple
Garter Stitch Scarf in lavender
Garter Stitch Scarf in variegated shades of blue
Garter Stitch Cowl in vargateted pastel shades of pink, purple, blue and yellow.