Try this...
select a.name as [Name]
,b.class_roll as [Roll]
,count(case when c.Attendance='true' then c.date else null end) as [Date]
from usera a
inner join students b on b.student_id = a.user_id
inner join att c on c.student_id = b.student_id
where a.subject_id='" + DD_subject_view.SelectedValue + "'
AND a.course_id='" + DD_Chosecourse_view.SelectedValue + "'
group by a.name,b.class_roll
order by b.class_roll