Try using it this way:
var list = from s in Staff
join sr in StaffRole on s.StaffId equals sr.StaffId
join r in Role on sr.RoleId equals r.RoleId
where r.Description == 'SpecialisedHealthManager' && s.PrimaryShm == 0
select new
{
StaffId = s.StaffId,
FirstName = s.Staff.FirstName,
Surname = s.Staff.Surname,
SamAccountName = s.Staff.SamAccountName,
RoleId = r.Description
});