The Nexus 5 has a micro USB port, so you can’t use its cable on the Pixel 3. All Pixels are powered through USB-C ports.
USB-C chargers have a wide variety of power output profiles, since they can be used to charge everything from phones to laptops. I don’t have a Pixel 3, but I do have a first-gen Pixel. And the first gen Pixel charger is great, capable of putting out more power than most other small USB-C chargers. I suspect the Pixel 3 charger is equally good.