Unless you're part of a union, you won't be making any kind of complaint to the Alberta Labour Board. Such a thing doesn't really exist anyway (Alberta Labour Relations Board does exist). If you are not part of a union then the Employment Standards Code governs.
It sets out the minimum standards that all must abide by. You cannot contract out of these provisions but you can contract for something better (e.g., more than minimum wage, obviously). There are exceptions galore to the minimum standards in the Code. Articling students are not entitled to overtime
There seems to be some merit to your argument that you are entitled to three hours pay just for showing up. You not following of policy may play in your former employer's favour. If it's true that you were not made aware of the policy at all then things might get a bit sticky. They're willing to pay you for that time so no worries.
Insofar as grounds for letting you go, troutman is correct, in my opinion, that your employer needs not provide you with any notice within the first three months of your employment. An employer can let you go for just about any reason during that time.
For greater certainty, contact Employment Standards directly:
Call the province-wide Employment Standards number toll-free by dialing 310-0000, then dialing (780) 427-3731.
... and let us know what they say.