how to solve this problem

Given three integers x,y and z you need to find the sum of all the numbers formed by having 4 atmost x times , having 5 atmost y times and having 6 atmost z times as a digit.

example say 4, 5, 6 occurs 1 time

The ans for the above example is 4+5+6+45+54+56+65+46+64+456+465+546+564+645+654=3675