In Mathematica a matrix is a list of lists so to enter a 3 x 3 identity matrix type:
{{1,0,0},{0,1,0},{0,0,1}} or more simply IdentityMatrix[3].
To get a dot product type:
{{1,0,0},{0,1,0},{0,0,1}}.{{a,b,c},{d,e,f},{g,h,i}}
or
Dot[{{1,0,0},{0,1,0},{0,0,1}},{{a,b,c},{d,e,f},{g,h,i}}]
In the Desktop version of Mathematica, keyboard entry or the Basic Math Assistant allows you to enter a matrix of the size you require with empty place holders but this doesn’t seem to be possible with Mathematica Online.
