FSBO seller, shows house to potential buyer, then get's contacted by a Buyer's Agent asking for commission amount

The buyer has the right to be represented by an agent. Chances are that the buyer is not comfortable not being represented and asked the agent to make sure that they're comfortable in the process.

That being said, you also have the right to not sell to someone using an agent, but that's a much tighter range of buyers and usually consists of investors.

You might want to consider proceeding with the sale however to get it through with this buyer. If you want to avoid paying the 3% then make the terms so that the buyer pays the agent the 3%. You're free to demand any terms you want, as long as they're agreed to and make sense for both parties. If they don't agree to that, then you need to decide if the 3% is worth it to lose or not.

Is this typical?

Yes. The buyer has a realtor to represent their best interests and part of that includes negotiating. Choosing to do FSBO is saving the 3% ($9,000) you would normally pay to the selling agent. As Jonast92 mentioned, you could decline to sell to someone using an agent or decline to pay the 3% as part of the negotiations. With or without an agent, negotiating a reduction in price, allowance for renovations, and/or splitting closing costs is typical and should be expected.

Negotiate with them. Make a counter offer. That's how this process works (at least in the US and several other countries).

You’ve not tagged or otherwise indicated that this applies only to the US, so here’s the answer from a UK perspective. The “buyer’s agent” is most likely a fraudster trying to insert himself in the process, because buying agents are almost unknown and when they do exist, they are always paid by the buyer.