https://www.theverge.com/2018/10/18/17996928/verizon-pixel-3-temporarily-locked-requires-activation

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.