All three sentences are correct and convey the same basic meaning. They all indicate that you can make a reservation for a ticket one month prior to your intended travel date. The choice of wording can vary, but the meaning remains consistent.

All are perfect. #3 is the most logically complete since the preposition has a direct object, "your travel date". If you want to be precise, say it this way. But, #1 and #2 are fine because the missing object is easily understood: "in advance of the travel date" "ahead of the travel date".

From a British English speakers perspective. The most common way would most likely be. "You can reserve a ticket a month in advance". Getting to know when to say "a' versus "one" is just a matter of exposure to the language and hearing it in use everyday. "a month" "one month" = are the same.

They are all correct but 2 sounds less natural to me. 1 and 3 are something a native speaker would say. Both ‘a month’ and ‘one month’ sound natural to me. Using ‘one’ emphasizes that you are talking about an exact time period.

They are all correct, but they would sound more natural if you just said: "... a month..."