The advice provided in stanza three fits in with the theme of the poem, which is to advise a young man of the qualities he has to strive for and acquire in order to become a morally upright and respected human being.
"Pitch and toss" alludes to a game of chance—as the two previous posts indicate. What the speaker suggests is that true moral value and courage lies in the ability to risk everything that has been gained and not complain once it is lost but to rebuild from scratch and regain what has been lost. Such an act suggests a paradox, for the speaker essentially states that any young man worth his salt will not cry over spilled milk. He will instead be happy regardless of the outcome of his actions and have the courage and determination to recreate what has been lost.
The mere act of risking everything to gain a greater advantage was, in the type of paternalistic society in which the poem is set, seen as a remarkable display of manliness. A man was supposed to be morally upright and loyal to his family, his king, and his country and give everything as proof of his commitment. Such a man did not make excuses for his failures but had the vigor, drive, and bravery to recreate and face whatever trials and tribulations life may throw at him.
In essence then "pitch and toss" is a phrase which symbolizes the vagaries of life. The term alludes to the uncertainty that one faces—nothing is guaranteed. The expectation as expressed in the poem is that if one should wish to be judged capable as a man, one should be able to deal with whatever destiny brings into one's path, and one should not be afraid of taking risks no matter what the perceived outcome might be.