Google's Android Open Source Project is the codebase where the Android magic happens before it's compiled and packaged as updates for devices worldwide, and that codebase has apparently seen mention of a 2018 Pixel lineup. Interestingly, the text in question was saying that there were some sort of concurrency tests that should only be able to run on the aforementioned 2018 Pixels and newer devices, which means that the tests could pertain to a new Pixel-exclusive feature, or a new processor generation or machine learning feature that's going to debut with this year's Pixel devices. This could be related to a previous commit wherein a new network-related hardware abstraction layer exclusive to the new Pixel devices is mentioned, which could mean network features and performance beyond anything yet seen in a consumer handset. Whatever the case, the reference to "Pixel 2018" instead of Pixel 3 makes it quite clear that we will be seeing these devices in some form before the end of this year. Other details and the usual seafaring codenames have yet to drop.
As stated above, this is not the first reference to the Pixel 3 lineup in the AOSP codebase, but it does confirm that the new devices are going to be revealed in some capacity this year. The most likely scenario is that they'll be unveiled at a special event in October, just like the Pixel 2 and Pixel 2 XL before them. Likewise, two different iterations are likely, but not a sure thing at this point - AOSP code seemingly has yet to reference the Pixel 3 or Pixel 2018 lineup in plural. Speculatively speaking, the new devices are likely to take advantage of the horsepower improvement in the Qualcomm Snapdragon 845 processor over the previous generation in order to roll out deeper AI integration in some form. If the phones come out in October, they will almost certainly ship out as a showcase for Android P, which is currently doing the rounds as a nameless developer preview for select devices. It's worth mentioning that this is the first year that the new version's pre-public releases have been available outside of Pixel and Nexus devices, which makes the possibility of new Pixel-exclusive features that much stronger.
Right now, almost nothing is known about the new Pixel lineup, aside from the fact that at least one device from it will be revealed and perhaps released this year. Even that's not an entirely certain thing, since unforeseen delays could affect the device, and with the usual late-in-the-year release window for Pixels, this one getting pushed into 2019 is not completely out of the question. The device may also debut alongside at least one and more likely three Pixel-branded smartwatch, a device that was revealed at Google I/O. New Pixel Buds are probably also in the works.