Episode 3.5 features two short segments titled "Matsu-jiru" and "Douteina Hero." 
 
Matsu-jiru - One day, the Matsuno boys are greeted by a morning different from usual... A special day for them begins. 
 
Douteina Hero - One day, Choromatsu discovers a group of college students enjoying a barbecue. But he feels inferior next to these college students who live in a different world. What's revealed is...
 
(Source: Anime News Network)

Episode 2 - Let's Get a Job / The Melancholy of Osomatsu

Episode 3 - Tidbits Collection

Episode 3.5 - Virgin Heroes

Episode 4 - Let's Become Independent / This Is Totoko

Episode 5 - The Karamatsu Incident / ESP Kitty

Episode 6 - It's a Birthday Party, Dajo / Iyami's Great Discovery

Episode 7 - Todomatsu and the Five Demons / Four / Going North

Episode 8 - The Calming Osomatsu / Totoko's Dream

Episode 9 - Chibita and Oden / Jyushimatsu Falls in Love

Episode 10 - Iyami and Chibita's Rental Girlfriend

Episode 11 - Christmas Osomatsu-san

Episode 12 - Year-End Special-san

The "Osomatsu-san" Year-End Special!

Episode 13 - Sanematsu-san / Girlymatsu-san / Accident?

Episode 14 - We Caught a Cold / Todomatsu's Line / Choromatsu-sensei

Episode 15 - Interview / The Life of Chibita's Flower

Episode 16 - Matsuno Matsunan / The Ichimatsu Incident

Episode 17 - Jyushimatsu Festival

Episode 18 - Iyami's Counterattack

Episode 19 - Period Drama Osomatsu-san / Choromatsu Rising

Episode 20 - Tell Us, Hatabou / School Matsu / Iyami's School

Episode 21 - Mahjong / Godmatsu

Episode 22 - The Star of Hope, Todomatsu / Final Sheeeh

Episode 23 - Kerosene / Dayon Tribe

Episode 24 - Totoko's Huge Panic / Letter

Episode 25 - Osomatsu-san, Such As It Was

Mr. Osomatsu JRA Special 2016

Studio Pierrot

Mr. Osomatsu Episode 3.5