My company brought me in as salaried with expectations of a 45 hour week EXCEPT during training, where I was hourly and was told I could NOT work overtime. When I received my paycheck, the salary was based on a 45 hour week, hence in my case, I was missing about $3/hr... is this legal?

You aren't clear on what you were told. If your salary is to be 950/wk, that's 40 hrs @ 20 and 5 hrs @ 30. If you are working 37.5 hours during training, your gross would be 37.5 x 20 = 750. Divide that by 45 (payroll semantics), you appear to be getting 16.+/ hr.
Is that an example of what's happening?

"When I received my paycheck, the salary was based on a 45 hour week". Not exactly sure of your question as to what pay was for-45 hrs? It is not illegal to base your salary on a 45 hour work week, they told you up front it would be. Does your company consider you exempt or non exempt?

Salary is not really "based" on anything, if you are salary, they may work you as many hours as they wish. 45 may be the average hours they will expect you to work.

So if you are salary, that is just it, your salary, and you are paid that amount each week, regardless of how many hours you work. Most salary employees in America work 50 or even 60 hours or more a week.

During the training, you should have set an agreed to hourly rate. It is often less than your salary when you begin full time work after training.

If you did not agree on a set hourly fee, for your training, then it is fair for them to pay you, the normal training rate.