Stack smashing detected : terminated Aborted (core dumped)

My array is of size 3x3 means i have values of index from 0 to 2 only. but when i traverse using for loop then why it is picking value of a[3][-3] on value of a[2][0] ???

and what is that error when i try a[3][3] it should give garbage value so why this error

*** stack smashing detected ***: terminated

#include <bits/stdc++.h>
#include <iostream>
#define vi vector<int>
using namespace std;

int main()
{
    char a[3][3];
    int pos, row, col;
    cin >> pos;
    memset(a, 48, sizeof(a));
    //row = pos / 3;
    //col = pos % 3 - 1;
    a[3][3] = 'X';
    //a[3][-3] = 'X';
    for (char *b : a)
    {
        for (size_t i = 0; i < 3; i++)
        {
            cout << b[i] << " ";
        }
        cout << endl;
    }
}

for a[3][-3] result will be output:

0 0 0
0 0 0
X 0 0

for a[3][3] result will be output:

0 0 0
0 0 0
0 0 0
*** stack smashing detected ***: terminated
Aborted (core dumped)

This is because of wrong arithmetic as @Kevin answer in comment that

a[2][0] = 2*3 + 0 = 6 bytes

and also

a[3][-3] = 3*3 - 3 = 6bytes

so both pointing to same value.