ok, perhaps there's something not right about the ports you've opened.
first of all, let me re-iterate that it's a really bad idea to open ports on your router for anyone on the internet to access your cams (even if you think you have decent passwords!)
That said, you do know that you must map DIFFERENT ports to each cam, right?
IE, if port X on your router's outside IP is forwarding to port X at your hik cam's IP, you cannot also have port X forwarding to another camera.
There's only one port X on the outside, so if port X forwards to port X on cam1, port Y can forward to port X on cam2, port Z can forward to port X on cam3.
if your router won't let you forward an external port to a different internal port then (a) it's lame and (b) you'll need to change the port on the camera as well.
Set port X to forward to port X on cam1, port Y to port Y on cam2, port Z to port Z on cam3 (after moving the ports on the cams themselves)
Hope that makes sense, and apologies if you know that already, but it's not clear from the info you've provided so far...