Would there be an EMC or cross-coupling problem due to 3
free-running oscillator SMPS running inside one box, often the
case that 3 would be togrther in close proximity charging 3
phones at the same time.

It's unlikely their transformers would have sufficient stray flux to produce
interference.

If you're worried, position two of them at right angles, and the third
not-in-line with either.

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