Stanza 3 is all about moving on. If Stanzas 1 and 2 were about the past, Stanza 3 is about "now."
Even the verb tense is all about the present: instead of saying "now she rises," which could make the "now" into a more theoretical sort of present, Angelou chooses to describe Africa's rising by using the present continuous tense. That's when you use a form of the verb "to be" with a gerund (a verb ending in -ing). It's a super-immediate form of the present tense. When is Africa rising? Right Now.
remember her pain remember the losses her screams loud and vain
And now the speaker turns to her audience, drawing them into the work of the present by insisting that they remember all of the things that Africa has suffered. Repeating the verb "remember" makes sure that we, as readers, will feel compelled to participate in the work the poem is doing.
This poem becomes the scream that Africa herself can't make. It's got readers (us) and a message.
We're betting that the speaker is certain that her message will be anything but "vain." Africa can't speak for herself? Well, no worries, friends. That's what this poem is for.
remember her riches her history slain
Here's the puzzle for this particular stanza: how do you kill ("slay") history? Our speaker doesn't offer many details, which makes these lines particularly haunting. Maybe it means that men and women were murdered as part of the slave trade. That would certainly be true. Maybe it means that transporting people away from their cultures is a form of genocide as well. That would also be true. In fact, there are lots of ways that history can be distorted, forgotten, or deliberately silenced. By refusing to specify which way Africa's history is slain, Angelou leaves the door open for all sorts of horrible potential silencings.
Once again, though, this poem steps in to offer a deliberate alternative to lost history. Sure, it's a literary re-creation of history – but that's a form of history just as much as the chapters you'll find in your social studies book.
now she is striding although she has lain.
You may think these last two lines are part of the same old routine – after all, we've come across some version of both of them before, right? We're back where we were at the end of Stanza 2, or even Stanza 1 for that matter.
Notice, though, the change in the wording of these lines. Africa is now "striding/although she has lain." Why is that such a big deal? Well, it suggests that things are changing. Sure, she once sat passively and did nothing as generations were captured and slaughtered. But that's all over now. Now she's moving and shaking and doing things despite her long history of silent victimhood. It's a new era, folks, and it's one that the speaker is pretty happy about.