Решение:
- Look at those black clouds! It is going to rain. (На основе предсказания по наблюдаемым признакам)
- We are having a party next Saturday. I've already sent the invitations. (Present Continuous для запланированного действия в ближайшем будущем)
- "I'm very hungry." "I will make you a sandwich." (Решение принято в момент речи)
Ответ: 1. is going to; 2. are having; 3. will make.