If you have an allergy towards crab and shrimp will you have an allergy with crawfish?

Probably. They are very closely related so there is a good chance. Not absolutely true, so it is worth testing. I'm afraid there are no commercial skin test reagents available for crawfish, so a RAST test is a good start. But even if it is negative, please talk to an allergist; you should first eat some in the allergist's office before trying some at home or in a restaurant. .
Shellfish allergy. It is common to cross-react to foods in the same family. Lobster, shrimp, crab and crawfish are all members of the crustacean family, so if you are already sensitive to shrimp and crab, you are more likely to be allergic to crawfish. However, you may not be allergic to mollusks, like clam, scallop and mussel, since they are in a different family, but you should be cautious if you are uncertain.