Generating a password

I have an application that when a form is submitted it generates a random 8 digit password. The password will be created form three fields in the form which are joined in to a single string. Them using the random number experession to generate 8 numbers that corresepond to a characters position in the string. The bit I am struggling with is the expression to get the nth number in the string. Can anybody advise on how to get the nth charchter in the string or even if there is a better way to do it .

Thanks