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The Google Pixel 3 XL has both wired and wireless charging capabilities but for wired charging the phone still supports standard USB Power Delivery 2.0 technology with its own bundled Google 18W charger. Let's see how the phone charges using the Google Universal Type-C Charger 60W which is one of the first USB Power Delivery chargers in the market that shipped with the Google Chromebook Pixel (2015).

With a fully discharged battery, we plugged in the Google Pixel 3 XL to the Google Universal Type-C Charger 60W using a USB Type-C to Type-C cable to observe charging. Immediately the voltage initialized to about 5 volts although it took a while for the phone to only start pulling current below 1 amps. At some point the phone briefly pulled current up to almost 2.9 amps but quickly resumed to pull much lower current under 1 amps while still at 5 volts. After the first minute where the battery has charged for a bit and the phone was switched on, a reset occurred which finally allowed the phone to charge at around 14.3 watts (5.1 volts @ 2.8 amps). While the phone was actively used, it pulled lower current to charge at around 8 watts (5 volts @ 1.6 amps). The phone then subsequently pulled even lower current to charge at only about 3.5 watts (5 volts @ 0.7 amps) when reaching 35 minutes of charging. More details here by downloading the trace captured by the GRL-A1 power analyzer. 

The Google Universal Type-C Charger 60W is able to deliver near to 15 watts as advertised to charge fine with the Google Pixel 3 XL using USB Power Delivery. 

Download the A1 software from Granite River Labs here to view the trace.