The myth about put the most selective column to the left is crap - sorry.
Your composite index will only be useful if you use the n left-most arguments, e.g. if you have the index on
(when, who, what)
then that index can answer question asking about
(when)
or about
(when, who)
or even about
(when, who, what)
but it cannot answer questions about
(who, what)
(since the left-most column isn't being used here).
That should be the point you consider - order the columns in such a way that you can answer the most of your questions with such a composite index.