What's he saying?
"Look in thy glass and tell the face thou viewest / Now is the time that face should form another;"
Look in your mirror and tell yourself that it's time you should have a child;
"Whose fresh repair if now thou not renewest, / Thou dost beguile the world, unbless some mother."
If you don't have a child that looks like you, you're being unfair to the world and the child's would-be mother."
"For where is she so fair whose unear'd womb / Disdains the tillage of thy husbandry?"
For there is no woman who would not want to have your child.
"Or who is he so fond will be the tomb / Of his self-love, to stop posterity?"
And there is no man stupid enough to die before having a child.
"Thou art thy mother's glass and she in thee / Calls back the lovely April of her prime;"
When your mother looks at you, it's like looking in a mirror; she is able to see herself young again.
"So thou through windows of thine age shalt see, / Despite of wrinkles this thy golden time."
In the same way, when you're old, you will regain your youth when you look at your own child.
"But if thou live, remember'd not to be, / Die single and thine image dies with thee."
But if you die before having a child, no one will remember your beauty.
Why is he saying it?
Sonnet 3 is one of the "fair lord sonnets," one of the first 126 of Shakespeare's sonnets, which are either addressed directly to or written about the effect of a young and strikingly beautiful man. It is also one of the "procreation sonnets," which focus on the fair lord's responsibility to have a child so that his beauty might be passed on for future generations to appreciate. Allusion to the story of Narcissus is apparent in Sonnet 3, in the fair lord's tendency to "look in thy glass."
Though he admires the fair lord's beauty, the speaker views the young man as selfish, too. This is because the fair lord seems to show no interest in bearing children, and thus "dost beguile the world." The speaker pleads with the fair lord, using his knowledge of the young man's vanity to try to convince him. Though he cannot stop his own eventual death, surely he cares about preserving the image he so loves staring at in the mirror? Therefore as the last line, the speaker warns, "Die single and thine image dies with thee."
The extended metaphor of farming runs throughout Sonnet 3. In lines 5-6, the speaker asks the fair lord, "For where is she so fair whose unear'd womb / Disdains the tillage of thy husbandry?" The word "unear'd" means "unploughed," and here is used metaphorically as a reference to sexual intercourse. Ploughing the womb and sowing it with a seed results in procreation. "Tillage" means the cultivation of land, and "husbandry" functions both as a reference to farm management as well as a pun on the state of being a husband.
The idea of a window is used as both a connection to the past and a barrier between the past and present. In lines 11-12, the speaker tells the fair lord, "So thou through windows of thine age shalt see, / Despite of wrinkles this thy golden time." The windows represent the eyes, through which the fair lord will be able to look upon his children, who will resemble him in his "golden time." But he himself will still be an old man with wrinkles, separated from his youth by unstoppable time.
The theme of the ravages of time is prevalent throughout Shakespeare's sonnets, and in the fair lord sonnets, it is connected to lamenting the fact that the fair lord's beauty will fade and he will eventually die. In this sonnet, the speaker is trying to convince the fair lord that time will pass and his beauty will fade; he will not always feel such pride when he looks in the glass. This unavoidable truth is hinted at in lines 7-8 when the speaker asks, "Or who is he so fond will be the tomb / Of his self-love, to stop posterity?" Here, "fond" means "foolish." It is extremely foolish to become "the tomb" of that which you love so much about yourself, which is beauty.