Stanza 3 Summary

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

That age is best which is the first,
When youth and blood are warmer;

The speaker divides life into several periods and says that the "first" (i.e., young adulthood) is the best because "youth and blood" are "warmer."
"Age" just means "period of time" here.
"Youth and blood" probably aren't literally warmer, but we often think of dead people as cold, so perhaps the speaker means something like "farther from death."
Alternatively, "warmer" might even mean something like "more vigorous and healthy."

But being spent, the worse, and worst
Times still succeed the former.

Youth is the "best" time of life, so the speaker says. Once it's gone, the "worst/ Times" follow.
"Spent" means "used up" or "gone."
"Worst/ Times" refers to the period after youth is "spent," so it most likely means old age or something to that effect.
We're not quite sure what to do with "worse." We might have to supply syntax from the previous two lines and read the line as "being spent, [that age is] the worse [rather than the best]."
That, however, doesn't make a whole lot of sense. It seems easier to read the lines as "the worse, and [even] worst / Times" will follow.