Isaac's Universe series

Anyone familiar with these? I ended up with 2 volumes of this series of short stories based by other writers but based on an outline of a universe provided by Issac Asimov.

But I ended up with Vol 1 and Vol 3

I started reading Vol 1 and actually wasn't exactly captivated, the second story so far (By David Brin) was better than the first (by Robert Silverberg) but I'm invested now and I'm curious how many volumes there are. Strangely I can't find a definitive list in my Internet searches. I've found Vol 2 so I know there's three but everywhere I find references they are isolated references for one volume or the other or for particular short stories within the series. Nothing that gives a full list of all the volumes. I'm guessing from this that there are only 3 but I'd like to find out for sure.