... whether to go to London or not #2 and #3 sound natural. No, "it's all my guess" makes no sense. I cannot determine what you are trying to say.

I didn't make up my mind whether I (would, should, etc - you need another verb here) go to London or not. I didn't make up my mind on going to London. OK I didn't make up my mind about going to London. OK Do all three sound natural? He went to India. He DRANK (one doesn't eat liquids) a cup of water given TO HIM by a stranger. He fell asleep. At that moment, EVERYTHING in his bag WAS stolen. THAT'S my guess. In the story, is "It's all my guess." used correctly?

Regarding the three sentences about London: I didn't make up my mind whether I should go to London or not. I didn't make up my mind on going to London. I didn't make up my mind about going to London. All three can be understood in context, but the first one sounds more natural with the addition of "should." The third one ("about going to London") is the most naturally phrased of the three. Regarding the story about India: The phrase "It's all my guess." is understandable, but in more common English, one might say, "That's just my guess." or "I'm just guessing." This suggests that the prior information is speculative and not based on confirmed facts.