Quatrain 3 Summary

Get out the microscope, because we’re going through this poem line-by-line.

Then, soul, live thou upon thy servant's loss,
And let that pine to aggravate thy store:

After freaking us all out about what happens to our bodies when our "lease" on life is up, our speaker shifts gears here.
Almost all of Shakespeare's sonnets have what's called a "turn" or a "volta." That's when the theme or the tone changes in a sudden or surprising way.
This one is more subtle than most but there's definitely a shift happening here. (The big tip-off is the speaker's use of the word "then," which means "therefore.")
Up until now, the speaker has been building an argument about why his soul is such a chump for neglecting itself and letting the body have its way.
Now he offers it some advice about how it can turn things around. He tells his soul to let his body ("servant") do all the starving/wasting away ("let that pine") so that his soul can increase ("aggravate") its own resources or treasure ("store").
Technically, souls can't amass treasure so this is a metaphor about building up a bunch of spiritual wealth that the soul can "live [...] upon."
How, exactly is his soul supposed to go about doing that? And, what's going to nourish it, precisely? (Is he supposed to pray more? Go to Church every Sunday? Volunteer at a soup kitchen?)
Let's see if we find out.

Buy terms divine in selling hours of dross;

Apparently, the soul has got to "buy terms divine." Awesomesauce. Wait, what does that mean?
"Terms" means a set period of time and "divine" is another word for God so, "terms divine" probably means for all of eternity.
According to Christian theology, all the good Christian souls get to chill with God (the "divine") forever and ever. You know, while the worms are busy munching on their corpses back on Earth.
At the same time, the word "terms" can also mean conditions or stipulations. So in a weird way, it almost sounds like the speaker wants his soul to cut some kind of a business deal with God, especially since the speaker tells his soul to "buy" his way into heaven by "selling hours of dross."
Of course, souls can't technically sell stuff, especially "hours of dross." No worries. He just means that he wants his soul to stop wasting all its time and energy doing a bunch of scummy stuff. ("Dross" is literally the scum that forms on metal while it's being melted down.)

Within be fed, without be rich no more,

Check it out, Shmoopers. Another feeding metaphor.
Now our speaker tells his soul to worry about nourishing itself ("within be fed") instead of being so worried about outward show (which probably refers all that expensive clothing and jewelry and stuff that he referred to at line 4).
This takes us back to line 3, where the speaker accused his soul of starving and wasting away while the body flourished.
Basically, he wants to turn things around.