How to select specific entries of a matrix?

How to select elements of matrix at the following rows and columns

1,4,7,10,13,16,...

In other words, select elements of matrix at first row and first column, fourth row and fourth column, seventh row and seventh column and so on...

Answer 1

[Matt J]

A(1:3:end,1:3:end)

[Ammy]

Thank you.