You encountered this error because there is a Table name before
Inner join
and after
on condition.. Try this one... you can use table aliases....
SELECT c11.workclass, c11.fnlwgt, c11.Age, c12.maritalstatus,
c13.education, c12.workclass AS Expr1,c13.workclass AS Expr2
FROM classify11 c11
INNER JOIN classify12 c12 on c11.workclass= c12.workclass
INNER JOIN classify13 c13 on c12.workclass = c13.workclass