Well not sure if you are teaching elementary school or not but go to naver and type in
위 베어 베어스 (We bare bears) 시즌 1 or 2 and you can get a bunch of 20 min cartoons that have perfect Korean subtitles.
Another good one is 그래비티 폴즈 (Gravity Falls) 시 즌 1 also 10 or 12 min clips to fill in time.
I know you asked for 3 mins clips OP, but I just use these to fill in the last 10 mins (if need, but so far not needed) and whatever we don't finish I just bookmark it and come back to it the next class period.
Hope this helps.