so, you still didn't dig up 3 diamonds in one click. i've got 68 luck and still haven't done that. are you sure you didn't mean gold nuggets or something else? even the rare occasion that has been reported as 3 diamonds in one click (by a 70+ luck digger), i have not heard for sure that it's true. what i've heard is that they "are pretty sure" they saw "you dig up 3 diamonds", but were never totally sure.
but to answer your original question as to why 3 or 4 don't get 3 or 4 times as much experience as 1... there's a large random-luck factor in digging (and stealing, and i'm guessing robbing) experience. i did take notes of over 500 digging clicks, and it appears that whether you dig 1 diamond or two, the average experience is the same. whether you dig 1 gold nugget or 30, that average is the same. now, the diamond average is higher than that of gold nuggets, which is higher than copper. but i found the number of one item that you dig has no effect in the experience you get. after all, you already get the benefit of getting more money from more items per dig... so there isn't double-benefit because there isn't more experience. hope that helps.
--jason