I think the answer is
It'$ not ₺he pride before the fall.
First Stanza - Pride is said to lead to bad fortune. So, It's good fortune.
It's not 'round mouths in afternoons.
First Stanza - It's not round mouths. So, it's pursed lips. Humor me, we'll call this a purse.
You may have heard it said by three:
Bounty - a generous gift; something freely provided - The 3 witches in MacBeth gave him a few warnings freely with their magic spell.
Do it in meta 'for toil and trouble.
Bounty - a premium or reward, (given for some act or action) - I'm sure putting a bounty on a question in META is not a good idea. It's pretty unnecessary.
Or if the charge is all done wrong,
When the bounty is offered for a bad deed, like for a hit
It's guaranteed for subtraction!
Then the person targeted will most likely be killed.
Ensure the sword's not just a blade,
Bounty - a payment made by a government, to a sailor on enlisting or to a soldier after a campaign - the sword could be a gift or reward for service, more than just a blade.
And within the forest it might be.
Fore - first in place, ... - So, assuming forest could mean first, the first stanza is a purse filled with good fortune (has money signs in it). So, the bounty could be in the purse.
It is relevant to this question because
The longer this question sits without a correct answer, the more likely it is to have a bounty put on it.