Twozaskars
Retro Guru
Credit to winjohn, no point anyone else writing it again.RD is pretty straightforward.
Set H limit screw so the top jockey wheel lines up with smallest cog and will not come out any further.
Then grip mech, pull back and push towards the wheel as far as you can and set L screw so it will not go in further than biggest cog.
People often think (not necessarily you, but some people) that those H and L screws are for some sort of fine tuning of the gears but they are there purely to restrict the maximum travel of the derailleur.
You say it stops at 3. With tension on the cable, either by slight force on the lever or grab it under the downtube and pull down, so that the mech moves toward the wheel.loosen the L screw (lefty loosey righty tighty) until it goes as far as needed.