Me_Blue There is no audio metronome built in to ST3 as far as I know, you have a visual metronome that is a rough guide to the beat of your song , but it is fixed to an average tempo of the song , reason is , ST3 does not recognise the beat of your whole song all the way through as it is TIME BASED, the beat setting in ST3 is an average beat of your whole song.
You need to record your CLICK track in a DAW and play it back in ST3 , use the visual click as a guide if you pause/mute your backing for a short period.
Perhaps ST4 will be able to generate a click , Peter is the one to fill you in on that.