Question

how do you know where are the pivot points in a matrix i know that 3x3 1's across but 3x4 and 3x7 are different pivot columns

Answer 1

Pivots actually depend on whether the columns are independent.

Answer 2

When you do elimination on the matrix you'll find that some numbers in the pivot position may go to zero, and you'll have to switch rows until you have a non-zero or move on to the next column because the previous column may not have a pivot.