The easiest way to expand a 2x2 is to select the matrix; you will note a black handle at the bottom-right hand corner; just click'n-drag to the larger 3x4 matrix.
per the help:
If you change your mind about dimensions you can always change the matrix dimensions. Just click inside the matrix on the right lower corner. The black square with a vertical and horizontal line will appear. With the left mouse button (mouse pointer will change the appearance) drag and correct the dimensions. You can do that even if the placeholders are not empty, https://smath.com/wiki/Print.aspx?Page=Matrices
May this be of Good Help; ⚜ Kenny Lemens, P.E. ᵂᴵ
"No matter where you go, there you are." -Buckaroo BanzaiHotkeys: https://en.smath.com/forum/resource.ashx?a=45771&b=2