3. My friends __________ (not to do) their homework now. They __________ (to play) volley-ball.

3. My friends are not doing their homework now. They are playing volley-ball.

В первом случае необходимо употребить Present Continuous в отрицательной форме, т.к. действие не происходит сейчас. Во втором случае - Present Continuous, т.к. действие происходит сейчас.

Ответ: My friends are not doing their homework now. They are playing volley-ball.