put them in sockets
no, this is what switches are for. If you put the FETs in series with the batteries, if you open one, they all open, so you are not switching one two or three battries into (again we ask) WHAT????? If you won't tell us your application, then everyone is just shooting in the dark trying to help you. We aren't trying to be difficult, but you are making it difficult for us to help you. If you are trying to power a circuit or load with 1.5, 3, or 4.5V, you are going about it the wrong way.