1) Press 's' on you're keyboard then click 'Fit' to make it fit perfectley.
2) To make a door right click you're brush that you wan't a to be a door and go 'Func_door' in the 'func' menu.
3) Elevators are a blind spot to me too but Rich Diesels tutorials covers that. Everything that works in JK2 mapping wise will work in JK3, but JK3 has a few more advantages - Like weather in MP.
4) To make a door open by a switch make you're switch texture on a brush, (The switch texture can be anything). Then make a thinner brush over the top and make the texture of it 'system_trigger'. Right click the trigger and make it a 'trigger_multiple'. Then press 'n' with the trigger still selected and tick the 'use_button' box. Now unselect everything. Now select the trigger and the 'func_door', and hold ctrl and press enter with ctrl still holded. Now there should be a line connecting you're trigger and door. You're done!
BSP it and test it and it should all work.
Hope this helps. I am using GTKRadiant 1.5.0.
Quake 2 player.
Real life player.