Most of the enduring classic cocktails comprise just three ingredients e.g. rum+lime+sugar=Daiquiri. A few, such as the Negroni consist of three equal parts.
The three-ingredient cocktail is the holy trinity of bartending so, starting with gin and vodka cocktails, we've set about collecting them in this area.
Truth is, most "three-ingredient" cocktails, including the two examples above, actually have four ingredients with ice being an integral part of a "on-the-rocks" serve, and/or contributing water to dilute the drink while also cooling during mixing. Hence, we have not included ice/water when counting ingredients.
We've also, in some cases, turned a blind eye to few dashes of bitters here by considering them embellishments rather than core ingredients.
It's incredible how many great three-ingredient cocktails there are.