How do you increase the mechanical advantage of a third-class lever?

Move the effort force closer to the resistance force

Imagine you are using some tongs to pick up some fruit from a bowl but the tongs do not provide enough force to hold and pick up the fruit. (perhaps the coefficient of friction between the tongs and the fruit is exceptionally small). You can increase the force by pinching the tongs closer to the fruit.

In this case you have moved the effort force (your hand) closer to the resistance force (the picking up of the fruit)

But then again you have lost some of the benefits of using a 3rd class lever, gaining distance at the sacrifice of force. 3rd class levers are purposefully designed to have small mechanical advantages.