Yes, it's the good old "Multi-tasking" fiction again!

As Jez says, no single processor can ever do more than one thing at a time; so-called "multi-tasking" systems are really just doing one thing at a time, but switching between different things so quickly that it looks like they are doing them simultaneously!

So you must either do as Jez suggests, and have some parallel hardware than really can sample all the inputs simultaneously, or just have your processor sample them fast enough so that it looks like they are being sampled simultaneously!

Of course, the "parallel hardware" could be (or include) a number of small, cheap "slave" processors...