Be sure not to put more than three ____________ of vodka into the mix.

The correct choice is "spoonfuls" because it is the appropriate plural form of the noun "spoonful." In this context, the sentence refers to measuring a certain quantity of vodka using a spoon. The term "spoonful" indicates a specific amount that a spoon can hold, and when referring to more than one of these measurements, the standard plural form is "spoonfuls."

The other forms do not fit the grammatical structure needed in this sentence. "Spoon" is a general noun and does not convey the specific measurement intended. "Spoonful" in singular form would incorrectly imply just one measurement rather than multiple. Lastly, "spoutfulls" is not a recognized standard term in English for measurements and is not applicable in this context. Therefore, "spoonfuls" effectively conveys the intended meaning of measuring vodka in multiples of a defined quantity.