TWO ORIGINAL SWITCHES - ONE three-way and one single cant figure out wiring for SAFE UPGRADE
On cables 2 and 3, black and white are hot and neutral, respectively. 

Each of cables 2 and 3 goes to a light.  

Those 2 facts seem in contradiction to each other, but they're not.  Power comes in at one of the lights, through this switch, onward to the other light, and then onward to points beyond.  


  which cable is my Line-In &amp; my Load-Out...


It's not wired that way.  Line-in and Load-out are happening in the same cable.  Line-in is the black, and Load-out is the red.  

Cable #1 is a 3-way "spur" to the other 3-way switch, which has only one cable interacting with it.  Red and black are travelers and I recommend you get yellow tape and re-mark them yellow on both ends.  White is the switched-hot for that lamp.  You MUST remark that some sort of suitable hot color on both ends.  I recommend red, since it's a switched-hot.  

As far as installing any smart switches, you must install the smart switch "master" at this location.  The only thing that can go at the other end is a smart switch remote.