Use speaker phone? Come on, we can't be giving people recommendations like this, it sounds like "Oh yeah... the S3 simply can't make normal phone calls so you have to use speaker phone". You're a cool dude Funky and I like ya, but responses like this might have a negative affect on new users.
I don't have any problem whatsoever with the phone selecting options while on a call. Did you go into the settings and make sure it's properly set to disable the screen while on a call? Sounds like for some reason the option might be off. I'll check on my phone where the option is and edit the post in a few minutes.
*EDIT - OK, tap the Phone icon and when it launches tap the menu button and choose Call Settings. In there is an option for "Auto Screen Off During Calls". Is that box checked? If not it should be.
Rev.