3 These flowers are beautiful. They..... expensive, weren't they. - No, they......

3. These flowers are beautiful. They were expensive, weren't they. - No, they weren't.

В данном предложении необходимо использовать форму глагола to be (быть) во множественном числе в прошедшем времени. Поскольку речь идет о цветах (These flowers), используем "were" и "weren't".

Ответ: These flowers are beautiful. They were expensive, weren't they. - No, they weren't.