Properties of pulsed corona streamers are measured and simulated in full three spatial dimensions (3D). Stereo photography is used to measure branching angles and to investigate whether apparent streamers reconnections are real. 3D simulations of two parallel streamers show that they can repel each other electrostatically, but that they also can merge due to photoionization. The electrostatic interaction of several streamers becomes evident through theoretical investigations of a periodic array of streamers.