Some Google app users are getting a third tab within the app that displays at the bottom of the app interface. The new tab is a search icon that allows users to complete a Google Search query without having to scroll to the top and tap inside of the search box. Although this new tab acts as a second method for searching for things within the Google app it's possible that Google is testing this tab out so it can provide users with a quicker way to access search if they're already looking at something else inside the app, though because it's currently in testing there is no guarantee that Google will end up pushing this feature to all users.
That should be even more apparent due to the fact that most users haven't even seen the two-tab interface that Google has had in testing with a smaller number of users for some time now. Since the two-tab interface has yet to roll out to everyone, the three-tab interface could end up following the same path. Whether or not Google decides to keep it also likely depends on how well the feature is received in the first place, and since it essentially does the same thing as what the search box allows, there is always the chance that Google may not keep the tab.
Then again, it could also implement other features as part of the search tab so that it does more than just allow the user to have an additional method for searching. When Google might open up these new UI changes for the Google app to everyone is still unclear with it being about six months or so since the two-tab interface first started to appear for some users. With it having been such a long time already, it's possible that Google may continue testing the new tabs interface for the Google app for quite a while longer. For those that may have been unaware of the tab interface altogether, the first two tabs that started showing up last year separate the Google app feed into various news stories and such that you've designated as information you'd be interested in, and a tab for upcoming information that would display details like package shipments, calendar events, and travel information.