Sum of points and rank according to points
Try this query

select a.*,b.points from table3 a
join(
select Sum(PointsUp-PointsDown) as points,t1.id
from table1 t1
  join table2 t2 on t2.Id = t1.id
where t1.ActivityTime BETWEEN '2015-01-01' AND '2016-01-01'
Group By t1.Id)b on a.id=b.id
order by b.points desc