Making a 3by3 matrix in Microsoft Excel is in fact very simple. You need only write numbers in each cell of any 3by3 block of cells. What has led to so much confusion regarding matrices in Excel is their functionality. A 3by3 matrix by itself provides no use unless you can either multiply it by another matrix, find its inverse or use it to solve a system of equations  all three of which are possible in Excel.
Multiply Two Matrices

Type two matrices in 3by3 cell blocks anywhere on the sheet. The matrices need not be adjacent.

Highlight an empty 3by3 block of cells and press the "F2" key.

Press the "=" key on your keyboard and type "MMULT(."

Highlight all the cells containing the first matrix; type a comma and highlight all the cells containing the second matrix. Type ")."

Press and hold down the "Ctrl" and "Shift" keys, and then press the "Enter" key to finalize the calculation and make the resulting product matrix appear in the 3by3 block of cells you highlighted.
Find Matrix Inverse

Enter numbers in any 3by3 cell block. No more than one row may contain cells in which the numbers are the same.

Highlight an empty 3by3 block of cells and press the "F2" key.

Press the "=" key on your keyboard and type "MINVERSE(."

Highlight all the cells containing the original matrix and type ")."

Press and hold down the "Ctrl" and "Shift" keys, and then press the "Enter" key to make the inverse of the initial matrix appear in the empty 3by3 block of cells.
Solve System of Equations

Write the equations in the system of equations you're trying to solve in the form of x + y + z = c. You can write it on a piece of paper or in your computer's notepad. You need the coefficients to build the required matrices. This is an example:
x + y + z = 10
2x + 2y + 2z = 20
3x + 3y + 3z = 30 
Fill a 3by1 cell block with the numbers to the right of the equation: 10, 20 and 30.

Setup a 3by3 cell block matrix by using the coefficients to the left of the equation. Type
1, 1 and 1 in the first row; 2, 2 and 2 in the second; 3, 3 and 3 in the third. 
Highlight an empty 3by1 block of cells and press the "F2" key.

Press the "=" key on your keyboard and type "MMULT(MINVERSE(."

Click all the cells in the 3by3 block. Type ")." Type a comma and highlight all the cells containing the 3by1 matrix. Type another closed bracket.

Press and hold down the "Ctrl" and "Shift" keys, and then press the "Enter" key to make the values for x, y and z appear in the 3by1 block of cells you selected.
Tips & Warnings
 Highlight multiple cells by clicking the first cell, holding down the "Shift" key and clicking the last cell.