Try to pick potatoes with similar sizes so they are ready more or less at the same time.
First wash the potatoes. Do not peel before hand or your potatoes will be too starchy.
Then cover the potatoes with cold water and add a teaspoon of salt, to make them tasty from the start. Place on high heat first and once the water comes to boil reduce to heat. Make sure the water is still smirring.
Do not cover the lid to make sure the potatoes are cooked evenly.
Check after about 10 minutes by sticking in a knife in the middle of a potato. If the knife easily slides in the potato, it means that it is ready. Boiling should take about 10-20 minutes.
When the potatoes are boiled enough, take them out of the pot, rinse with cold water and peel them. Then mush them, either with a mashing tool or a fork. Split the mashed potato into three.