Did you mean to say it is 3 cm? Or 3 in.?
If it is 3 in., then it is definitely NOT a Red Claw Crab. I kept a tank of Red Claw Crabs for several years, they only grow to a little over 1", maybe 1.5" at the most, at full adult size.
I don't have the resources to help you directly, but I will...