How to ensure notified block is confirmed?

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How can i make sure the notified block is confirmed and related transactions of block is final?
I'm trying to monitor my wallet for any deposits, Someone told me to make sure block is verified i need to monitor older block of blockchain with difference of 3, which mean i always need to get the height=best_height - 3, Is this idea correct? How can i trust the notified block?
My problem is when i try test with simnet chain params, i will be notified just after generation of any new block with the height of newest one. should i do any configuration for getting a block differ than newest block height?
I would be appreciated if you guide me with your suggestions