The purpose of brushing you teeth is to remove dental plaque from all surfaces except between teeth. In other words, the floss is the toothbrush for the inter-proximal area ("between teeth"). If you brush your teeth but do not floss, you are leaving a lot of plaque and bacteria which can cause cavities and gums diseases.
It is important to brush your tongue since it accumulate dental plaque also, creating bad breath.
In conclusion, all of them are "a must" if you want to be able to chew something more than just mash potatoes when old.