Whatever size you choose for the biggest "pebble", make the next size 1/5 the size/diameter.
If you consider the pebbles as perfect spheres, the smaller one will just fit in the gap in a tetrahedral packing.
(Well I've got to start with some assumptions, haven't I.)
I can't see how to calculate the third size, so tomorrow I'll try a practical approach.
John
Edit - By the way, if you consider shaking up a dry mix, then sucking in the epoxy with a vacuum, the dry shaking will separate the sizes, with the biggest ones coming to the top !!