My thoughts would be that if you were simply loping the pattern it may not be so bad. But the owner probably doesnt want her running barrels yet.It would not be beneficial for a young horses joints or mind to be worked heavily like that. On the other hand... at 3 I'd think if the horse is being ridden, it should also be lightly cantered to learn correct balance and how to carry itself. But this isnt something the horse needs drilled into it.
You mentioned having a year until your other horse is ready to ride. A year is not very long, so although you can definitely progress, I want you to know you will not be able to ride her hard just because you have a year to work her. If you already have one in training, i'd suggest looking for a more seasoned horse to ride until you are training your other one.