The Pixel 3 and Pixel 3 XL phones being sold by Verizon will not support SIM cards from other carriers unless they are first activated on Verizon’s network. A Verizon spokesperson confirmed this to The Verge, pointing to a new policy that the carrier implemented earlier this year to dissuade thieves from targeting its own retail stores and those of authorized resellers.
Once a Pixel 3 (or any of Verizon’s other current phones) is activated, the carrier will automatically unlock the device “overnight” that same day, according to the company. So there’s not much of a wait to get around this, but there is a wait all the same. Remember that Verizon is the exclusive carrier partner in the United States. So your only alternative — and the one folks on other carriers should definitely take — is buying direct from Google. Google seems to have a good handle on stock (with most storage and color options available within a week), but shipping isn't immediate.
This policy presents a hurdle for anyone who was hoping to walk into say, a Best Buy, and try to buy a Pixel 3 outright at full price to use on their own carrier. (Most Best Buy stores probably wouldn’t even allow this, for the record.) But it’s also something to be aware of if you buy a new-in-box, unactivated Verizon Pixel 3 from someone online.
You’ll have to find some way of briefly activating it on Verizon before it can be used elsewhere. I’ve asked if popping in a friend or family member’s Verizon SIM card for a minute or two would be enough to tell Verizon’s systems to trigger the unlock.
The original Pixel and Pixel 2 were released before this policy took effect, so they were immediately unlocked and compatible with most major networks.