It looks like A is a 2D matrix with a variable number of rows and 3 columns. If so, does not use length for getting the number of rows as it will return the number of columns if you have less than 3 rows. Use size(A, 1) to get the number of rows.
No loop is needed to get your result:
angle = asin(A(:, 3) ./ sqrt(sum(A.^2, 2)))
Your issue is that you want to do elementwise division so you need ./ instead of /.
I've also simplified your square root expression.