With the original larger matrix you want the rows and columns from being "A" and the resulting 3x3 being "B",
A = B(1:3, ... % to specify first 3 rows
end-2:end); % to specify last 3 columns
Since I'm assuming you want this to be a function, and B could be anything (including a matrix less than 3x3, it would be a good idea to have an error check.
function A = first3last3(B)
m,n = size(B);
if (m<3) || (n<3)
error('Input matrix is improper size');
end
A = B(1:3, n-2:n);
end
