Season 3 is the show's third and the final season of Clarence (series). The actual date as well as the actual number of episodes of the season is unknown, the season started on February 10, 2017, starting with the episode "Valentimes".
The 3rd season has been announced quietly without any official news about the show season rewenal, it follows after Season 2 ended with 39 episodes total, while the other episodes, such as Sumo Goes West, Clarence for President, and Clarence Loves Shoopy, were originally part of Season 2 episodes and was originally going to air between November 28th to December 2nd alongside with Merry Moochmas, but were postpone indefinitely, which apparently these episodes were seperate completely, Around between Mid to Late January, the Showrunner announced that season 3 will start on February 10th, with two new episodes (Sumo Goes West and Valentimes), following a week after Pizza Hero (a season 2 finale episode)
On March 30, the show was not mentioned nor listed at the Cartoon Network Upfront 2017-18, an indication that is official that Season 3, is the last season of Clarence (series)
Also the voice acting of the season is completed  as of March 31, 2017,  as Spencer Rothbell stated that the remaining sets of episodes will air through until 2018. 
As said for Season 3 had a small significant changes, between Sumo and the other two kids, who had changed schools, and also will feature a few guest stars cameo, and will also episodes that focuses on some characters and more holiday specials.
On October 19, the series production had been finalized its completion, the series will end with over 133 episodes.
It’s school election time at Aberdale Elementary, and Jeff is once again determined to become Class President. However, like previous years, he’s unsuccessful in connecting with other kids, that is until he sees a shoe-in puppet President, the lovable Clarence!
Clarence plans to throw the sleepover to end all sleepovers with decorations, pillow fights, and games, but there's one thing missing - guests; Clarence wants invite all his friends, but finds that they might need some convincing.
During the storm, Clarence goes out to get Jeff a raisin drink. While gone, Jeff has to stay in the house with Chad and Mary, and eventually goes crazy about having the house cleaned. They later set out to find Clarence.
While trying to get to California for vacation without telling Clarence, Miss Baker gets trapped in the store with Clarence, the Mayor, Meg (the substitute teacher) and one of Jeff's moms. The four get suspicious of Meg and try to leave the store.
As the storm gets worse, everyone gets stuck in the school's gym and eventually fight. After Clarence gets them to get along, the big flood occurs and everyone works together to block the flood from entering the school.
The boys go the Aberdale Community Pool for a day of wet fun; Jeff wades in the shallow end; Sumo tries to conquer the diving board, redeeming himself from last summer's bellyflop; Clarence sneaks past the lifeguards into the hot tub.
After surprising Clarence with Martial Arts classes at the Aberdojo, Mary gets talked into joining the class herself. She loves the slow-paced lessons in discipline and control, but Clarence just wants to have fun and kick butt.
Clarence finds a fast friend in JT, who shows him some cool new music, a new outlook on life, and a sweet rat-tail; Clarence leans into the rat rail lifestyle, discovering a whole new side of Aberdale he never knew existed.