For any i, someVector[i] has 0 elements, which means that someVector[i][j] does not exist.
And the most outer one should be initialised to iMax, not kMax.

Code:
const int iMax = 50;
const int jMax = 20;
const int kMax = 10; 
int x = 5; 
int y;

vector<vector<vector<int>>> someVector(iMax, vector<vector<int>>(jMax));

for (int i =0; i<iMax; i++) {
    for(int j = 0; j<jMax; j++) {       
      for(int k = 0; k <kMax; k++) {        
           y = k * j * i * x;         
           someVector[i][j].push_back(y);  
       }
    }
 }