Because cm is open source, so people are working together(its a huge collaborative project). But anybody can use the project source to build a ROM so several different people build the ROMs while including different smaller modifications in order to make one that suits the needs of different users. But bugfixes and the likes are added upstream and is generally added to all the ROMs.
If I'm right, the main difference that they are for different phone models. (one is for the international and US GSM versions, one for the dual-SIM, and the third one for some custom model of a service provider)
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