Are all IMU's involved in regular flying incl. Auto-Tune?

just wondering, if a FC has 3 IMU’s, are all IMU’s involved in regular flying - including auto-tune - or is my assumption correct that first IMU is the primary and the others are there just for failsafe reasons?

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Thanks for that.
I was scratching my head as I was sure I’ve seen something in that regards a few years ago but couldn’t find that information anymore.