How to increase breastmilk during 3rd trimester?

It'll only start after birth. If you want to build a fridge/freezer supply then the more you empty your breasts, the more milk it makes. So if you want to produce more you need to feed, pump or express more often. But if you don't intend to build a supply then don't express/pump too often unless engorged or you can become an oversupplier.