That's a seriously oddball motherboard for a current platform like LGA 1151. Not many people need 3 PCI slots any more, at least not in consumer applications. (For niche markets like industrial control you still see PCI or occasionally even ISA slots, but that's a whole 'nother thing.)
What additional functionality do you need that the onboard peripherals aren't providing, and which can't be added via the pair of 1x PCIe slots? (I'm assuming the x16 is already taken up by a GPU since you said it's a gaming PC.)
If you've already filled up the existing PCIe slots and need additional functionality that could be handled by a classic PCI card, my suggestion would be to search Amazon and eBay. You're probably going to be limited to mostly off-brand or secondhand cards, since (as you've correctly surmised), PCI is now obsolete.
Nostalgia isn't what it used to be.