select Area_ID
from Table1
where Supp_ID in ('a', 'b', 'c')
group by Area_ID
having count(distinct Supp_ID) = 3
Or, to clarify where 3
comes from:
declare @Filter table(ID nchar(1) primary key)
insert into @Filter values ('a'), ('b'), ('c')
select a.Area_ID
from Table1 as a
where a.Supp_ID in (select t.ID from @Filter as t)
group by a.Area_ID
having count(distinct Supp_ID) = (select count(*) from @Filter)