It might be worth trying code below, as it seems to limit the number of "j" lookups required. A nested-loop join didn't prevent the extra lookups -- for whatever reason -- so I forced a hash join.

select a.aid, i.*, isnull(i.spoint ,j.spoint) as spoint

from #a a

left join #i i on a.Aid = i.aid

left hash join #j j on (i.aid is null) and a.Aid = j.aid