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 !!
It's like doing jigsaw puzzles in the dark.
Enjoy today's problems, for tomorrow's may be worse.