I've heard all 3 devices are pretty fantastic in their own respect. I've only owned a S8 for about a week before I switched to a G6. Pixel = Amazing photos and speedy updates straight from Google OnePlus 3T= Solid device and solid value. Definitely the cheapest of the bunch. The S8 is the new kid on the block with the fresh kicks and best aesthetics. Samsung pay will open up your mobile payment options significantly and the S8 has a best in class display.
Actual owners may chime in with their own recommendations but I'm not sure you'll find someone who owns all 3.
Just my 2 cents as I know nothing of your preferences or what's important to you in a device.