Italian meringue
Italian meringue is cooked: the whites are whipped while hot sugar syrup, boiled to roughly 118–121 °C, is poured in. The hot syrup partly denatures the protein, making the mass stable and safe without further baking.
The syrup goes in as a thin stream down the side of the bowl, clear of the whisk, or it will spray caramel around the sides where it sets in lumps. After the syrup is in, the meringue is whipped until it cools to around body temperature — otherwise it will collapse when folded into anything else.
It is the most stable of the three. It is used where the mass will not be baked: mousses, soufflés, meringue-based buttercreams, and the topping on tarts that gets torched. For macarons the Italian method is considered more predictable than the French, though it does require a thermometer.