It is based on the late Fujio Akatsuka’s original Osomatsu-kun manga, which features the Matsuno siblings as kids, and are all in love with the same girl.
The original manga ran from 1962-1969, and received several TV anime adaptations in the 60’s and 80’s.
Mr. Osomatsu follows the Matsuno Sextuplets as adults.
And much like previous seasons, Studio Pierrot will once again be animating the series.