Pixel 3 and Pixel 3 XL phones sold by Verizon do not support SIM cards from other operators unless they are first enabled on Verin's network. A Verizon spokesman confirmed this to The Verge and pointed to a new policy that the carrier implemented earlier this year to put off theft from targeting its own stores and authorized dealers.
When a Pixel 3 (or any of Verin's other relevant phones) is enabled, the operator will automatically unlock the device "overnight" the same day, according to the company. So there is not much hope to get around this, but there is a wait all. Remember, Verizon is the exclusive transportation partner in the United States. So your only option ̵
This policy gives an obstacle to anyone hoping to enter, say a Best Buy, and try to buy a Pixel 3 directly at full price to use on their own carrier. (Most Best Buy stores will probably not even allow this for the post.) But there is also something to be aware of if you buy a new in-box, unactivated Verizon Pixel 3 from any online.
You must find some way to enable it on Verizon before it can be used elsewhere. I have asked if you pop in a friend or family member's Verizon SIM card for a minute or two, would be enough to tell Verin's system to trigger the unlock.
The original Pixel and Pixel 2 were released before this policy came into effect, so they were immediately unlocked and compatible with most major networks.