Join Date: May 2007
Location: Fairbanks, Alaska
You should never have to change bits from the snaffle if you are comfortable with the one you are using and you are not competing in shows or events where certain bits are required. I am still riding my 5 year old paint in a loose ring snaffle; I have to switch over to a curb next year for showing because this is the tack requirement for his age, but I will continue to ride in a snaffle outside of the show ring.
Curbs are much more capable of being harsh because they have a shank, which gives the rider leverage, hence more strenth with less effort over the horse's mouth. Many also are unbroken, which will actually stiffen the horse.
If a snaffle works for you, you can ride your horse until she is an old lady in it. And....in my opinion, 3 years old is too young to switch over - I am sure many people have done it, but training is usually not entirely solid at this age - so why comlicate things by changing bits. If she is accidentally overcorrected with the more severe bit, she may become shy/resistant to the bit, and have continual problems with it in the future (become hard on the bit, a head tosser, etc.). You can only increase severity of bits so many times before it makes no difference, so why start increasing and running that risk at age 3 if not necessary.
I would do what it sounds like your gut is telling you.