Taking a turn in 3rd gear

Not sure if i'm in the wrong here was out with my trainer in my 2nd week of roll-off training and got thrown out of the truck.Was taking a right hand turn on to a service entrance of a shopping center couldn't see around the corner because of a 3 story parkade and slowed down and shifted to 3rd gear and took the corner.Well he proceeded to freak out and insisted those turns be taking in 4th.I told him that for the situation i felt comfortable taking the turn in 3rd,well to make a long story short he insisted they be made in 4th and if i didn't like it i could get out of the truck which i did.Now i'm a pretty new driver just having my class 3 for about 6 months and wondering was i wrong?

 

Thanks for the input i was always told to just drive the truck at speeds you feel comfortable at and not let someone else dictate how you operate the vehicle,since ultimately i'am responsible for anything that happens.

 

I also can't remember taking to many turns in 3rd gear either but it was a blind corner and my view of the road, pedestrians and oncoming traffic was blocked so i slowed right down and down shifted .Tried to explain to him my reasoning for doing what i did but he couldn't care less.Just kept insisting i should of taken it in fourth and tried telling me i was damaging the truck by taking the turn in 3rd even though it wasn't lugging and it slipped right into 3rd.This also the same guy who on the very first day gave me crap about downshifting to much to slow down, he said that's why these trucks have brakes.