Anyone know the name of this book series?

Back when I was in middle school, I read this fascinating book series that centered on the lives of a group of kids. The series consisted of 3 books I believe, and had a lot of scientific influence.

In the first book, the kids were young and I only remember a little...they meet this strange old lady and travel to another dimension to stop some evil entity...

In the second, I remember some of the kids had to shrink themselves to microscopic size, and travel inside a dying boy to save him....

The third book focused on time travel, and contained many different stories....

Sorry if a bit vague, this has been killing me for weeks now...