different sampleus time for three ADCs

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That’s a great idea, I’d recommend Microchip C-30 for exactly that reason among others.
PBP is very well documented, however you’ll notice all examples of command use are also examples of exactly how not to write a cyclic program.
ie. a program that gives the illusion of simultaneity.
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If it’s a cyclic program that will be broken if delayed, have you considered an external ADC chip? Not that I’ve used one.