What do You want to do with the signals?
Most multiple A/D Converters have only one internal A/D converter unit and use an internal Multiplexer to connect it to more external Pins.
So You can only convert one sine signal after the other.
When only want a voltage measuring that's no Problem.
But when You want to figure out the exact phase of the signals, You get an phase shift at the A/D conversation results.
That is possibly not torable.