filling the data in gridview by using join query

i have three tables

table 1:
users

userid, username, password


tabel 2:
roles

userid, groupid


table3:
groupid,name

i want to show a username from table1 and name from table 3 in grid view.
i wrote a query as.

SqlDataAdapter da = new SqlDataAdapter("select users.username,groups.name from users,groups inner join users on roles.userid=users.userid inner join groups on roles.groupid=users.groupid", MAconn); 

but i am getting an error

System.NullReferenceException: Object reference not set to an instance of an object. at Default2.bind()