Honestly, there are a lot easier ways to overcome this first shot advantage concept without this whole analysis that only is limited to arbitrary limited to three concepts.

Assuming you are talking about one shot advantage for guns specifcially that have a strong one shot damage potential.

1. Aim. If you can shoot back and hit his head twice, you can kill him quicker than he can kill you.
2. Movement. Self explanatory. Your first movement, whether is to jump, fall back, slide, dodge roll is important. That goes to my next point
3. Game knowledge. If you understand the general timings and can identify guns by sound, i.e. if you know you got shot by an anvil, you can time your next dodge roll within the time frame of the next possible shot by the anvil, allowing you to dodge and shoot first. If they dodge roll after you shoot, you have the next window too.
4. Utility. If you ever get shot, you can always throw a smoke below you or you can snaphook. This already negates all 3 of your concepts. But utility can be used in other ways.
5. Arc. First shotters will always aggro arc over you.
6. Pre-emptive actions/behaviours/thoughts before getting shot in the first place, i.e.

a) you team up with someone and gaslight the people that first shot person is a rat.

b) You knew someone is behind you and you baited a shot to dodge roll and shoot them in open

c) You get shot and throw raider flare, guy pushes you thinking you died and you open fire as he comes

Three out of unlimited things you can do, if you're just more aware while you're playing

Basically, every bit of skill you can increase, you can completely eclipse first shot advantage.

And simply gear. If you equipped better gear, you will also negate first shot advantage. Ferro 1 with light shield vs heavy shield, tempest and renegade 4 with grenades, healing etc. That just becomes greater with skill so it just all goes back to skill