"How are you charged with three counts of first degree murder when you only kill one person?"
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Stupid prosecutor tricks. You're thinking mathematically, not lawyerly. I am not in the legal field, so read this with a big grain of salt, but I did read an explanation of this and this is my recollection.
Basically, prosecutors charge two different types of murder charges for every instance of murder to give the jury two opportunities to convict the accused. The prosecutor wants the most punitive judgement possible in order to pump up their resumé, but they'll throw in the lesser charge in case there's a "holdout" in the jury or if the entire jury won't convict on the nastiest charge. If you recall, James Holmes was charged with 24 counts of murder in the Aurora theater murders even though only 12 were murdered.
This is the first time I've ever heard of 3 times the charges per actual count of murder.