Tear the greens into 2- or 3-inch pieces. Heat a large saucepan over medium heat and add the water, olive oil, and kale. Cover and let the kale wilt for 1 to 2 minutes. Add the watercress and mustard greens and allow them to wilt for another 1 to 2 minutes. Finally, add the spinach and let wilt for another 1 to 2 minutes. Drain any excess water, add salt and pepper, and serve.