Basics

Dynamically create an array of array:

void doforloop(unsigned char **buf,int x, int y)
{
    unsigned char *key = new unsigned char[y];
    for (int i = 0; i < 3; i++) {
        std::memcpy(key, buf[i], 64);
    }
    delete key;
}

int main() {
    unsigned char **ptr = new unsigned char *[3];

    for (int i = 0; i < 3; i++) {
        ptr[i] = new unsigned char[64];     
        randombytes_buf(ptr[i], 64);
        ptr[i][0] = (unsigned char)0xFA;
        ptr[i][1] = (unsigned char)0xCE;
        ptr[i][62] = (unsigned char)0xBA;
        ptr[i][63] = (unsigned char)0xBE;
    }
    
    doforloop(ptr, 3, 64);
}