You can do other things during the day, but your MITs should be what you need to get done. At least one MIT should be goal related. The other two are usually work/school related, but they don't have to be. This concept is fantastic. You will get the most important tasks done and out of the way, and your goals will be moving forward.
Try to get as many MITs done in the morning. If you wait until after school/work, then you will end up pushing them off even further. Doing your MITs in the morning gets them out of the way, and the rest of your day will be a piece of cake.