It's not dangerous so long as you and your horse are trained and experienced enough to do it. Your trainer must think that you're ready. If you're nervous about it, tell your trainer that you'd like to do the 2'9 a little more before going up to the 3 foot.

2,9 is nothing really but a big canter stride
if you ride well No if you don't it ok
But if you only ride 2'9 your in deep trouble
I jump 4 foot at a trot with stirrups but at a canter with out
so a 3foot oxer is not much higher then 2'9 but it depends on how wide the oxer is
tell us that ok

But if you dont have a seat dont do it !! you should be able to ride with out at 2'9 all the time perfect ,,all the time
if not NO
3 FOOT IS REALLY EASIER
BUT ITS NOT IF YOU DONT HAVE A SEAT
have you ridden in a chute with out hands ? and no stirrups ?
have you jumped with no hands ?

Your trainer is looking for you to get hurt