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Most trilogies or series follow chronological order, but there's no requirement. Do whatever serves your story. As long as it's clear to your reader what's happening when in relation to other event...
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#4: Attribution notice removed
Source: https://writers.stackexchange.com/a/23496 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#3: Attribution notice added
Source: https://writers.stackexchange.com/a/23496 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/