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Study strategy aside, CFA registrations for August 2021 are now closed, so you might not be able to carry out your exam plan.
As for your original question, I don’t think it’s advisable to try and pass CFA L1 in 3 months off the back of your FRM knowledge.
We recommend 6 months preparation, and even if you assume a pure 40% overlap, that’s 3.5 months, and that’s assuming you’ve completely mastered the relevant bits of FRM.
Obviously you’re free to try (registration issues aside) but I don’t think it would allow you to maximize your pass chances.