I've got a 4 speed and I'm pretty sure they use the same indicator rods (the chain thingy), I'm fairly certain you can't use the 3 speed chains (I wish you could cause I snapped mine today). http://www.oldbiketrader.co.uk/ has spares but at £40 for the whole assembly, it's eyewateringly expensive, so try this guy: http://phoenixhubgearrepairs.co.uk/
I've got a 4 speed and I'm pretty sure they use the same indicator rods (the chain thingy), I'm fairly certain you can't use the 3 speed chains (I wish you could cause I snapped mine today). http://www.oldbiketrader.co.uk/ has spares but at £40 for the whole assembly, it's eyewateringly expensive, so try this guy:
http://phoenixhubgearrepairs.co.uk/