Madagascar may be adopting a three-person presidential council.
So, if the agreement is confirmed (and allowing for its being only transitional), we can add Madagascar to a small list of countries ever to have had such an institution. There are also Iraq (where the council is chosen by parliament, but has suddenly burst on to the world scene as a relevant institution), and Bosnia-Herzeogovina (where it is elected, in a semi-presidential system. And then formerly Cyprus and Uruguay (in presidential systems, i.e. directly elected and head of government).
Did I miss any? I suppose there have been others, at least on an interim basis.
There is, of course, the granddaddy of plural executives–Switzerland.