Muskie, Taimen, and Walleye have all recently been referenced together in the Android Open Source Project, which would seem to further suggest that Google will indeed have three devices coming to market at some point in 2017. A post from today coming from Treehugger Robot mentions all three phones that look to be part of a test fix for known issues that were present on the Pixel and Pixel XL, which could be pointing out that all three phones were being used to test the fix for the Pixel and Pixel XL before actually pushing the fix for those devices.
Having all three devices mentioned alongside the Pixel and Pixel XL further solidifies that these three new devices are the next upcoming Pixel devices from Google for 2017, all three devices having first been mentioned as the second-gen Pixel phones as of earlier this year. There isn't any telling details that have been spotted in the AOSP gerrit but just the fact that Muskie, Walleye, and Taimen have seemingly been testing a fix that is the Pixel phone issues means that all three devices could mean that they are similar to the Pixel and Pixel XL in some way or another.
This seems to be the first time that all three of these devices were mentioned at the same time in a single AOSP gerrit, though Muskie has been mentioned in AOSP before on its own, as it was seen in AOSP back on April 4th, and since then it doesn't look like there have been any new discoveries of these devices. Originally Google was rumored to be launching follow ups to just the Pixel and Pixel XL with codenames Muskie and Walleye, but then a third device was later rumored to be codenamed Taimen. Google isn't expected to be unveiling any new hardware to replace the Pixel and Pixel XL until later this year in the Fall, so it isn't likely that anything will make an appearance over the next few days as Google rolls through their annual developer conference, though they are now rumored to be announcing a new standalone VR headset at the event.