Code

 Header...
#define M 3
 int main(){
   ... 

»  int n;
»  float *p[M];// This is the array of pointers

»  printf("Size of each vector?\n");
»  scanf("%d", &n);// Find the row size
  
»  for ( int i=0;i<M; ++i) 
    pi=xmalloc(n*sizeof *p[i]);// Allocate rows
  
»  p[0]1=3.14159;
  // Use p for something 
  
  // Advanced point: resizing the arrays
  // feel free to skip this.
»  printf("New size of each vector?\n");
»  scanf("%d", &n);// Find new the row size
  
»  for ( int i=0;i<M; ++i) 
    pi=realloc(pi, n*sizeof *p[i]);// Resize the rows
  
  // Do something else

»  return 0;
}