You want to use the find-Nth-pattern in SQL
Select dealerMobile, totalCost, deliveryDate
FROM
(
select ROW_NUMBER() over (partition by dealerMobile, totalCost, convert(date, deliveryDate) order by dealerMobile, totalCost, convert(date, deliveryDate)) as rownr,
dealerMobile, totalCost, deliveryDate
from tbMain
where dealerMobile='set the wanted number here'
order by deliverydate desc
) as rows
where rows.rownr = 1