A good start is 1 Lemon, 1, Lime, and 1 Orange. Or 1 large Grapefruit. Chop the fruit into 1" chunks -- skin, flesh, pips and all. Place in a blender of food processor and spin into a rough puree with bits of skin showing.
Transfer fruit puree to large microwave safe glass bowl. Add the same measure (cup for cup) of sugar -- white, brown, turbinado... your choice. Stir to combine well. Taste. If too bitter, add more sugar. Too sweet add half a pureed lime.
Nuke in the microwave on High, for 10 minutes, in two 5 minutes stages. Stir and taste between stages.
After 10 minutes, carefully ladle into clean Screw-top jars. Cool before lidding and sealing. Will last in the fridge for months.