QBASIC to display Greatest among 3 numbers using SUB

DECLARE SUB GREAT ( A, B, C )
CLS
INPUT "Enter any three numbers"; A, B, C
CALL GREAT ( A, B, C )
END

SUB GREAT ( A, B, C )
IF A > B AND A > C THEN 
PRINT " The greatest number is A"
ELSEIF B > A AND B > C THEN
PRINT "the greatest number is B"
ELSE
PRINT "The greatest number is C"
END IF 
END SUB