By joining tables you retrieve multiple tables of data using linq like below
var results = (from a in a1
join b in a2 on a.userID equals b.userID
join c in a3 on a.userID equals c.userID
select new
{
a.userID,
a.firstName,
a.lastName,
b.Email,
c.Phone,
c.Address
}).ToList();
Hope this helps