The same motif begun in the first sonnet continues in "Sonnet 3." The speaker urges the young man to marry and have children in order to pass on his beauty.
In this poem, though, the speaker emphasizes the joys that having children may bring the young man as he gets older.
Perhaps the most striking metaphor in this poem is the one that occurs at the beginning of the third quatrain (line 9): "thou art thy mother's glass." The word glass here means mirror. The beauty of the young man is a reflection of the beauty his own mother once possessed when she was younger. The speaker is thereby telling the young man that he preserves the memory of his mother's beauty. And if he were to have children, then they would reflect his own beauty as well.
The idea of the passage of time is also emphasized in this sonnet. The speaker tells the young man that when "age" (line 11) and "wrinkles" (line 12) come to him, he will have the beauty of his child to remember his own beauty when he was younger (his "golden time").
The sonnet ends in a manner similar to the first, with the imagery of death and the idea of waste (if the young man does not have a child).