Your query should looks like:
SELECT DISTINCT b.*, t.*
FROM bag AS b INNER JOIN txnd AS t ON b.IDColumn = t.IDColumn
WHERE b.bodchr = 'abc' and b.bloc ='a2b' and b.bcls = 'n' and b.bqty>0 and t.tddt >= DATEADD(day, -3, GETDATE())
More about Date functions:
Date And Time Functions (T-SQL)[
^]
You can use something like this too:
...
WHERE t.tddt BETWEEN DATEADD(day, -3, GETDATE()) AND GETDATE()