Detecting Multiple Lasers
A diffuser (e.g., a piece of frosted glass) can be used to scatter light from a large target area toward a sensor mounted some distance behind it. You'll need to do the calculation (or experiment) to determine whether enough light reaches the sensor from all points on the target area, and with different incoming light angles.

As far as detecting multiple sources, I would recommend encoding each gun with a unique pattern of pulses (rather than a DC output), and having the target count how many distinct codes it is receiving at any given time. Sort of an optical CDMA.