A few years ago I work for a company and they asked for volunteers willing to come in and work a half day on a saturday.
5 guys volunteered, including myself and a friend of mine.
I received rides from my friend to and from work back then.
My friend let my boss know that he (we) wouldnt be able to make it in at 6am which was the typical start time, and that we would be in at 7 instead. The boss agreed.
The next day we both showed up at 7, and all of the time cards except the 3 already working were removed from their typical spot. I then went up to the boss' office to sign in.
The boss informed me, that they werent in need of our work today as the other 3 had already had the load handled. He apologized to me, as he knew it wasnt my reasoning as to why we showed up @ 7.
My friend and I were quite upset about this, as we both gave up a good friday night inorder to make it to work the next day.
My friend and I thought about enquiring about the 3 hrs pay, but decided it was best to simply write it off and just forget about it.
I see your point in asking for the 3 hrs, but sometimes it's best to evaluate things, and pick your battles wisely.
From my understanding of the law the boss could let you go anytime he feels within the 3 month period.
You might have a case for the 3 hrs pay, but imo it wouldnt be worth the hassle. I'm sure it would take more than 3 hrs out of your day fighting to get it.
BTW you have officially scared me as I was thinking about getting a job as a ground crew man.
although I see how this situation could have been avoided, care to PM me which course you were working at?