You’ll want to add mana you want to double into your mana pool (tap your lands, etc…), pay three mana for the ability, then whatever is left after paying that three to activate will be doubled.

• 12/1/2004: Doubling Cube’s ability is a mana ability. All untargeted activated abilities that add mana to players’ mana pools are mana abilities.
• 12/1/2004: The “type” of mana is its color, or lack thereof (if the mana is colorless).
• 12/1/2004: Any restrictions on the mana in your mana pool aren’t copied. For example, if you have {W}{W}{B}{5} with no restrictions on it in your mana pool and {U}{U}{U} that can be used only to cast artifact spells, you’ll end up with {W}{W}{W}{W}{B}{B}{10}, {U}{U}{U} that can be used only to cast artifact spells, and {U}{U}{U} that can be used for anything.