Here's some basic differences between those guns, many new players who depend on one weapon only (either one of those 3) miss out on a lot and will never understand this game if they don't open their eyes.
LG is good to catch out incautious players in open areas. Look at what I wrote, every single word and it will make sense.
Rockets are much better in non-open areas and very close range, the higher level you come the harder it will be to hit perfect rockets, because ping influences close range rockets a lot and people know how to dodge and confuse the opponent (ALL WHILE SHOOTING BACK :D).
Rail gun only has two ideal uses: High distance and peek shots. What did I write? I wrote rail gun is good when other weapons have a big advantage, rockets travel too long and can be dodged/avoided easily, lg can't even shoot that far. Peeking: you already know where your opponent is, you look around the corner (ideally you switch to rg just now because it would have made that annoying sound and you'd be catched out, and shoot once and dodge back again, or you might be catched out big time again due to the long reload time).
RG is also good for finishing shot, providing that u know that enemy has less than 81 hp and u have nice aim. In close quarters SG may be more suitable because of shorter reload time. But RG is good finisher in all situations.
How can u dodge hitscan?) U can evade enemy crosshair, thats all :)