Put 1 of 3 batteries the "wrong way" to prevent accidental turning on?
If you have two batteries and they have precisely the same voltage then placing one backwards will effectively cancel out the voltages and no current will flow.

However, batteries aren't like that. The slightest difference in voltages mean that current will flow. Not as much as normal, but some will still flow. And that means, if the lamp is turned on, dead batteries - just not as fast.

Introduce a big imbalance, which is what you do with 3 batteries (2:1 voltage ratio) you end up with considerably more current flowing round your circuit, which means flatter batteries quicker.

Add to that the fact that you have current flowing backwards through one battery (even with just 2 batteries) and you are really asking for trouble.

So all in all turning one battery around is a ReallyBadIdea™ and should be avoided. Instead remove one battery entirely, or insert an insulating material between two batteries or between a battery and its contact.

Alternatively fix the switch so it doesn't keep turning on.