Here is my reasoning. #1 very unlikely to get caught as long as you keep your mouth shut. #2 Easier to get away without being apprehended, even if seen (could always change vehicles when the news reports a hit on yours). #3 Helps solve all sorts of logistics and intelligence problems. Could have someone unrelated to the actual shot pick locations, different drivers (that would he screw up possible witness identification). You could frequently change vehicles or firearms.
My gut feeling is that #3 is the answer because I think in #1 & #2 there would be some pattern to the victims. #3 would have a pattern to the victims only if the group had an agenda to put forward. OTH there is a pattern to the victims; they are so far all U.S. citizens/residents that are not dressed in appropriate Muslim garb. My bet is that this is a terror cell or some very sick individual.