I, too have a 3 year old who is very petite (only 37" tall)
and struggle with the dilemma of buying her a ticket
at both WDW and USF/IOW.
We made the difficult decision to go ahead and pay for her,
since she'd probably blurt out "I'm three!" if asked and
just because we felt it was the right thing to do.
(I'm not flaming those who choose the other, though!).
If height was used as the requirement for ticket buying,
I feel it would be much fairer than the age requirement.
Children under 40" really can't do as much
as those who are taller.
(Of course, I'm only 5' 2", so I don't think I hit 40"
until I was around 5 years old!)
It's a hard decision to make ...
Just follow your gut and do what you can live with!
(I know I'd be worried about my daughter busting me,
as well as her older brothers noticing the ticket signs
and asking why their little sister didn't have a ticket.)
Enjoy your trip!