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Do you value the continued functioning of your knees?
With all due respect, since you don't even know which one is the DIN screw, you should take them to a shop and get them adjusted correctly. If you bought the boots from them and they're a legit store they should do it for free.
Sorry for the double post but yes, 3mm is a big enough difference to have a significant effect on how your boot stays in the binding. In this instance, as your boot is 3mm smaller, you won't have enough forward pressure for the heel DIN to be effective.