You can't divide until you change the string in your text box to a number. Try formatpercent(csng(textbox3)/100). Csng will change the value to a single. Now you have to be careful that the user puts something that can be changed to a number in there. If they don't there will be an error. You can use the isnumeric funtion for that in an if statement. Error trapping is also the hardest part of using text boxes.
Also, usually you'd use text property of the textbox function to get the value (textbox3.text). I'm not sure if you're using this or not.
Dave