insert into mytable (columns)
select somevalue, somevalue, a.id, b.id, c.id
from
othertable1 a
cross join othertable2 b
cross join othertable3 c
where
a ... condition
b ... condition
c ... condition
insert into mytable (columns)
select somevalue, somevalue, a.id, b.id, c.id
from
othertable1 a
cross join othertable2 b
cross join othertable3 c
where
a ... condition
b ... condition
c ... condition