This is not a great sign.
Verizon being the "exclusive" carrier for the Pixel 3 and Pixel 3 XL is a bit of an annoyance because it reduces the options for getting the phones in a store and with incentives from other carriers. But now it looks like that exclusivity is more restrictive than in the past — Pixel 3s sold by Best Buy are seemingly SIM locked to Verizon. Early units being sold today (the first actual day of retail availability) from Best Buy aren't working with non-Verizon SIM cards.
To be clear here, Best Buy technically only sells a "Verizon" Pixel 3 and Pixel 3 XL. But if we were to follow the past experience of the original Pixel and Pixel 2, these "Verizon" phones were actually identical to the unlocked models and could be used on any carrier of your choice — the only hurdle was getting Best Buy or Verizon to sell you one without a Verizon account.
But yes, it seems that Pixel 3s sold by Best Buy are SIM locked — its product pages even say so. Compare that to Best Buy's listing for the "Verizon" Pixel 2 that clearly lists that phone as being unlocked, which it was. I was tipped off to the issue by a reader (thanks, Alex!) who had zero problems walking into a Best Buy and buying a fresh Pixel 3 XL without a Verizon account ... but was unable to get it to work with either a T-Mobile or AT&T SIM. Putting a Verizon SIM in it, the phone worked fine — but it complained again as soon as it had another U.S. SIM inserted, so this isn't just a "first-time setup" requirement.
Typically, the only reason why a Pixel would be SIM locked is if it was being sold by Verizon — or Best Buy as a Verizon reseller — on a multi-month financing plan. When the phone is sold full-price with no contract, Verizon account or financing plan, we would typically expect that SIM slot to be unlocked. At this point, the information we have is that this is not the case when the phone comes from Best Buy. Verizon can, of course, unlock any phone that has a SIM lock to its network. Whether a Verizon representative (in store, online or on the phone) would be willing to do that for you when you're not a Verizon customer is another question entirely.
For now, this means you should buy from the Google Store if you intend to use another carrier.
Thankfully, even if this is the by-the-book rule that Best Buy and Verizon are now following, this isn't a catastrophic problem ... so long as you know what to do. You can still buy the Pixel 3 and 3 XL from the Google Store, including with 24-month no-interest financing. Those phones will never be SIM locked — and heck, you can even use it on Verizon if you wish. You'll just have to accept the trade-off of not being able to get any incentives Best Buy and/or Verizon are offering with purchase.
For now, this means that you should stay away from buying a Pixel 3 or 3 XL from Best Buy or Verizon if intend to use it on a carrier other than Verizon. That is, until we get this whole situation sorted out.
I'm trying to track down the details on this, and have reached out to Google for clarification on the situation. This story will be updated with any future information as soon as it's available.