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Like parakleet said, we'd need a bit more info. However, since you've been approved for the SPG and the Green, I'd say you have a good shot at the Plat. The requirements are pretty much the same for all their charge cards. Can't hurt to try since its a SP if declined.
Yeah, you should not have a problem if you are a current cardholder. I got my MB platinum about 2 months ago. I had PRG, hilton, and BCE already. all four within the last 8 months with 28 EX inq and still got it. My EX FICO is around the 750 so YMMV. btw, the 50k MR points hit right after I met the 3k spend.