Tevul Yom, Chapter Four, Mishnah Three
This is the third mishnah in which we learn that something that has third degree impurity does not defile hullin.
A trough which is a tevulat yom, one may knead dough in it and cut off the portion for hallah and bring it near and call it by name [as hallah]; for it [the trough] has third degree uncleanness and third degree uncleanness is clean in hullin.
The kneading trough itself is a “tevulat yom”it was immersed in the mikveh to cleanse it, but the sun had not yet set. The person taking the hallah is pure, so she can simply remove the hallah, bring it near the dough without allowing it to touch the trough and call it by its name. Since the person is pure she doesn’t need to first arrange it on a vessel that cannot receive impurity. And as we have learned before, since the trough has third degree impurity, it doesn’t defile the dough itself, which is hullin.