Google Pixel 3 - Cannot Stop Security Updates
Unfortunately this this the proper way to initialize a newly bought Android device.

Google usually releases every month a new security update and each security update has to be installed individually, and often after the update installation a restart is required. 

Hence if you get a device that for example was produced 6 months before you have to install up to 6 updates one at a time.

In this case the Pixel 3 has been released in October 2018. If the device you got was produced at that time up to the time of the question up to 8 updates have accumulated and each update has to installed one-by-one plus reboot. 
Hence the first hours after switching on a new Android device you are busy installing security updates, which usually requires several buttons to be pressed at the correct time.