In the third stanza of "If," the author continues building his list of "conditions" for being a man. Specifically, in this stanza he addresses the importance of endurance, even in the face of difficulties and loss.
In the first four lines, he talks about how a person must face the loss of riches:
If you can make one heap of all your winnings
And risk it all on one turn of pitch-and-toss,
And lose, and start again at your beginnings
And never breathe a word about your loss ...
Even in the face of losing everything, a "man" must be able to get back on his feet and "start again," even if it means going back to his "beginnings." What's more, he needs to not complain about his loss to others - rather, he must stoically persevere through this set-back, not dwelling on the loss but looking toward the future.
In the next four lines, the situation or form of difficulty is more open-ended, but the need to endure is the same:
If you can force your heart and nerve and sinew
To serve your turn long after they are gone,
And so hold on where there is nothing in you
Except the Will which says to them: Hold on!"
Even when a person feels that his "heart and nerve and sinew" have left him, and he feels spiritually, emotionally, and physically weak, he must keep going. He must "hold on," even when the only thing he has left is the Will to say hold on.