If she's just turning three then I would wait to start barrels on her. My mare will be four in May, and I could not dream trying to start her yet. I agree with spastic dove in that you will make her break down by starting her to soon. Barrel racing is a very active sport for horses, and can really take it's toll on their legs if they are started too soon. If you want your horse to be good at it, then she needs to be taken very slowly.