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Hi, I'm Mike G and I'll be assisting you. One way to achieve the desired results would be to install a double pole switch at each fan having one side control the interior fans and the other for the booster fan. When either was on, the booster would also be on. Having both going to the booster would not produce a feedback.
If all 3 fans are on the same circuit there won't be an issue with feeding from either or both of the double pole switches. You'll need a switched wire from each location to the booster, being on the same circuit the neutral is already there. The existing feed to the booster is what you need to check as to its proxcimity to the switches.