If you have already done the application for all three and have been approved, it is likely in the Amex system that you have opened the accounts. Amex restricts you to earning one SUB in your lifetime for each card, because you have opened them already, this will be your only chance. In my experience, once the account has been approved, if you do not call within a few hours to cancel, the card will stick to your lifetime Amex history.
Here is what I would do:
1) Dont cancel any of them, the green and PRG each waive the AF the first year so you only have to pay the Plat.
2) Hit 5k MSR on the Plat first so you can get an easy 60k MR
3) Assess how much spend you have remaining in the 3 month timeline, if you think you can spend another 2k, hit MSR on the PRG to earn the 25k SUB. If you only think you could hit 1k more, then I would focus on the green card instead for the 10k SUB.
4) If you have hit the 5k on the plat, the 2k on the PRG, then work on the 1k for the green.
Do these in the order you are 100% sure you can hit MSR. For example, it doesn't make sense to go for 2k spend on the PRG instead of the 1k spend on the green if you only have 1.5k in spend left in your budget.
If you do hit MSR on all 3, you should still not cancel the green and the PRG until the end of year 1. Amex can and will claw back the MR points you earn if you cancel within the first year. Once the AF posts at the beginning of year 2, you can cancel then but I would first check if there are any retention offers.
Only spend what you know you can pay off each month because these are all charge cards. If you have some savings and you want to hit the SUB on all three, think of some bills that you might be able to prepay before the 3 months are up ie. insurance, cell phone, utilities. If you get desperate you could even buy a few gift cards that you could use in the few months following the 3 month period. Only do these if you have the money to pay off the monthly bills though.
As far as free spend goes, the airline credit from the platinum does count towards MSR. I would look into buying some airline giftcards to easily wipe $200 from the spend. The $550 AF will not however count towards the MSR, so in reality you have to spend $5550 on the Plat to get the SUB.
In the future, before doing Amex apps, ask google or the forum for the best SUB for each card. You will find from other posts that the Plat has 100k SUB offers and the PRG is at 50k with an incognito app. You left 65k MR on the table with the SUBs you got.
Good luck 
Edit: It looks like you did an app for the 25k/3k PRG offer. I assumed you had done the 2k spend. I'm too lazy to fix what I said above but you should be able to think it through while you read my post.