How to use multiple LDR to trigger a led?
The easy way is to put the LDRs in parallel, as @thece suggests. This may cause some cross-influencing (ambient light on one LED influeneces the trigger point of the other LEds).

Slightly more involved: repeat the circuit LDR-R2-Q1 for each location, and connect the collectors of the transistors (R1-LED only once, LED connects to all 3 transistors). 



For a very crude lasertag you might simply put LDRs in parallel, but I am not sure that hitting a single LDR (on a moving target!) with a laser (very small beam) is feasible.

Practical laser-tag systems use the laser only for the show effect: the real targeting is done with a (focused) IR beam, and the detectors are TSOP-style receivers for (modulated!) IR light. Very efficient, cheap, and high ambient-light suppression. It is exactly the way a TV IR remote works. And the IR signal contains the identity of the gun, so the victim's hardware can detect who hit him.

Fun note: the desire to build a laser-tag system is what dragged me into micro-controllers some 20 years ago. (The PIC 16C84 had just become available, bot no cheap proggers, so I had to make one, and no cheap HLL, so I had to make one, and not available in my country, so I started a webshop...) Dave Bodger's laser tag FAQ is still online!