HI ,,,,
Please try the below query
with cte_ex as
(select distinct studId,studName,(mark1+mark2+mark3) as sumt,ROW_NUMBER() over (order by (select 1 ))as rowno from tmpstd)
select A.studId,A.studName,A.sumt from cte_ex A
join cte_ex B on A.rowno=B.rowno
where A.rowno=2