How to implement three stacks using a single array

Question

I came across this problem in an interview website. The problem asks for efficiently implement three stacks in a single array, such that no stack overflows until there is no

Answer 1

Split array in any 3 parts (no matter if you'll split it sequentially or interleaved). If one stack grows greater than 1/3 of array you start filling ends of rest two stacks from the end.

aaa bbb ccc
1   2   3
145 2   3
145 2 6 3
145 2 6 3 7
145 286 3 7
145 286 397

The worse case is when two stacks grows up to 1/3 boundary and then you have 30% of space waste.