Hertl or Thornton depending on the hot hand. Although I'm one of the few people on this board who think Hertl is more effective as a Winger then a Center, especially when paired with Thornton.
If Hertl is on Thornton's wing, you have no 3C. And in reality, if Hertl is on Thornton's wing then you have no effective 2C because Couture is actually more effective these days on the wing.