In an extra large stockpot, add 12 quarts of water and bring it to a boil. Then add in the sliced apples. Let the apples simmer in the boiling water for about 15 minutes. Then remove them from the pot.
Now add in the strawberries, raspberries, and blueberries. Let the berries simmer for about 20 minutes.
Then remove the berries from the pot and stir in the sugar. No need to boil it. The kompot should be hot enough to dissolve the sugar. You may want to give it a taste to see if it needs more sugar according to your liking. The tartness of your fruit will determine how much extra sugar you will need to add.
Let the compote cool completely, then strain it through a fine-mesh sieve and refrigerate in a pitcher or a few glass bottles. Keep the compote covered with lids to preserve freshness longer.