Off the top of my head, I'm thinking NULL should have worked if the accepting table column allows NULL, and in any event if it didn't, that's a whole nother error message.  It seems more likely to me that you have spaces in your source.

Either way, the following ought to take care of it:

If the receiving column allows NULL:

INSERT INTO tableB (FieldNameB)

  SELECT CASE ISNULL (FieldNameA, '')

           WHEN ''

           THEN NULL

           ELSE CAST (FieldNameA AS DECIMAL (3,0))

         END

  FROM tableA

If the receiving column does not allow NULL:

INSERT INTO tableB (FieldNameB)

  SELECT CAST (FieldNameA AS DECIMAL (3,0))

  FROM tableA

  WHERE FieldNameA IS NOT NULL

    AND FieldNameA != ''

    AND FieldNameA LIKE REPLICATE ('[0-9]', LEN (FieldNameA))

T-SQL has an ISNUMERIC function, but for my tastes it is far too tolerant of non-numerics.  T-SQL needs an ISREALLYTRULYNUMERICHONESTTOGOD function.