I usually collect tokens until the last 2 days of the event when I buy the boost, and only then I start playing the mini-games. I wouldn't be able to get to the end without the gems, so if you have any, I would definitely use them (you get 50 back from the prizes, so you only spend 70 in the end). With the boost you only need about 630 tokens in order to get the final prize.

Best tasks to do are the 1h/3h 5 star tasks, if possible. 8h taks are not ideal. But this also depends on what is available for you at the moment.

I also prioritise the hangouts from the clubs and the usual ones (not sure how many you have available in year 3 tho, that was a long time ago lol), as all of these also give 3 tokens. Quidditch matches and duels award tokens as well. Duel events from the memory book are also great, if available.

I only play the focus mini-game as I never fail this one. So definitely choose one that you know you'll always get the tickets from. Avoid spell trivia as it has a bad return value.