2004 Oldsmobile Alero Replacing fuel injectors?

I took my 2004 Olds Alero to a repair shop because I smelled gasoline fumes in the passenger cabin. Their diagnosis is that three of the six fuel injectors need to be replaced at an estimated cost of $900-950 ($600 of this is parts). An acquaintance who is a mechanic told me he's never replaced three fuel injectors at once and very rarely replaces them at all on GM cars. What could cause three fuel injectors to need to be replaced at once? Should I be worried about the other three? What else should I ask the shop before authorizing the work?