How to use a selfie stick remote trigger with iPhone 3G?
The generic Selfie Stick button emulates the volume down button action on the iPhone. Initial iterations of iPhone(3G,4,4S) did not use the volume button as a shutter.

iPhone 5 and above introduced the volume down button as the "shutter key"

Alternatively, [Camera+][1] built the feature into their app back in 2011 for this purpose. Try the Selfie stick with the Camera+ app and it should work.

Incase it doesn't work, Open Safari and type camplus://enablevolumesnap into URL box and the feature will be enabled.