In this chapter, which describes how the Animals take over the farm and begin the process of doling out the work load, Boxer, the plough horse, is described as the hardest working of them all even though all animals did their part to the best of their abilities. Orwell describes Boxer as follows:

Boxer was the admiration of everybody. He had been a hard worker
even in Jones's time, but now he seemed more like three horses than one;
there were days when the entire work of the farm seemed to rest on his
mighty shoulders.

Boxer often does the work of several animals. He volunteers to do extra work, he makes arrangements to get woken up earlier than anyone else so that he can work a longer day. In short, he is the picture of dedication doing his part and then some so that those who are more limited in their abilities might have their needs met as well. He is altruistic and in it for the good of the group, not for personal glory. His motto was always

I will work harder

and he lived this motto daily.

The hardest worker on Animal Farm in Chapter 3 or in any other chapter is Boxer, the horse.  Boxer's main characteristic is that he is a very hard and dedicated worker.  In fact, one of his mottos is "I will work harder."

We can see how hard of a worker Boxer is in Chapter 3 specifically as well.  For example, we are told that Boxer works so hard that he seems to be three horses instead of just one.  We are told that he makes a deal with a rooster to wake him up half and hour earlier so he can get in extra work to help the farm more.