Instagram is testing a way to let you preview your photo grid without using a third party app،
Instagram is testing the ability for users to preview how a post will look before publishing it, putting aside the days of posting a photo and praying that it doesn't conflict with your grid of perfectly organized photos. This new grid feature is currently only available to a small number of Instagram users using the latest version of the app.
Source | Pocket lint
In this test, users have the option to “Show Preview” when publishing, just before hitting “Next” to make the publication official. This intuitive tool gives users the power to visualize their post's location in the grid before clicking publish.
No official announcement has been made yet regarding the rollout of this feature and there is no official timetable for widespread availability. However, based on past update patterns, we can assume that a full rollout could take place in future app updates, while A/B testing progresses. In the meantime, Instagram users can anticipate the grid preview feature with justified excitement.