The 3rd volume of the on-going Fire Emblem Engage manga will be releasing in standalone book format from 4th July 2024.
Above is the cover featuring princes Diamant and Alcryst, illustrated by Kyou Kazuro, via Shounen Jump.
This new volume includes 184 pages. Currently, the manga has 16 chapters (and a prologue), which you can read here (the first 4 and last 2 chapters are free to read). New chapters are added around the start of each month.
The manga is (still) only available in Japanese, although you can find fan translations online.