5 to 10:1 is a loose rule of thumb, but it's hard to say what that means in this context. You'd have to add wort until you had the cells in a solution at a density ~ the same as the starter had, then multiply that out by whatever ratio to figure out how much to add.
But if your starters are anything like mine in terms of how much gets left behind in the flask, adding a liter should be fine, and step it up from there.
Ultimately though, I don't know if 3 liters of 1.020 will get you enough yeast for a pitchable quantity, it depends on how much you're starting with and what you plan to pitch it in. But you'll be ahead of the game, and can always add more steps until you have enough slurry.