Heat packs and a visit to the chiropractor is what helps me. Quite often sciatica is caused by a twist in the pelvis which results in the nerve being pinched at worst or irritated.
Make sure you interview them first to be sure that they practice gentle methods, do a good massage to warm up the muscles first before they manipulate you and that they have experience with PG women.

Being your third bub you will find your pelvis is more susceptible to the relaxin so be super careful lifting the kids and putting them in and out of the car. Also when doing housework, especially vacuuming!