In this article a coordinated approach to 3D exploration of large uncluttered areas with a team of flying robots with constrained payload is proposed. Coordination is used as trump card for an effective exploration in feasible time and to overcome problems related to the limited computational power and autonomy of the robot platform. In particular, a 3D exploration strategy based on a combination of local and global information and a 4D motion planning are used for achieving 3D coverage of completely unknown environments.