Everybody has different morals. Morals are based on personal experience/preception. Someone whos morals state that people deserve to die, choosing to kill 3 over 1 would be the morally right choice to make -for that person-
The premise is subjective. Instead of moral, it should be ethic.
If the same logic is repeated, you would have to kill the three people you saved too. If you keep doing this, you will kill everyone on Earth. Therefore, the best solution is suicide.
Imagine if you were in the waiting room at a hospital waiting for a friend. A doctor comes over, kills you, takes your organs, with those organs he saves three peoples lives.
Its different now, all depending on the fact that it would be someone physically doing something to cause it
If you don't do it, it's not really your fault, nobody has to know about the deal, and if you kill someone its on you and they will probably ignore the 3 people and arrest you for the 1 because our court system is flawed in such a way that allows this to happen.