There are two distinct types of stylus - those which simply replicate a thin finger, and those which can respond according to angle and pressure. The former are cheap and can be used with any touch-screen phone, the latter are far more integrated into the device but work really well with apps like Photoshop which uses virtual brushes.
The reason why some reviewers use a stylus is because it obscures the screen far less than would dabbing at it with a finger (with hand attached!). So, yes, you can use a stylus with the G3, but not an 'intelligent' stylus.