Besides an raduino board I have a 3v 600mA power source and a 3v 350mA high power LED. I plan to run the led at 2.5v only and 200mA using PWM. Not sure if 2n2222a is okay to drive the LED, would it run hot?
From what I have read I can't use any MOSFET i.e. IRF510 or 2n7000 due insufficient Vgs voltage.
You do not have any choice at what voltage you run an LED at. It all depends on the materiel is is made from. For a power LED like this you need a constant current supply. You can set this to 200mA if you want.
So first of all you need to know what the Vf ( forward voltage ) of your LED is to see if 3V is enough even to start it glowing.
Anyway do you mean 3V or 3.3V which is more normal.
Any example that I can use as a startup point, on that web page all is pure theory.
Now you have me puzzled. I don't know what you consider pure theory but that site is exactly the opposite.
As a start you can take the last diagram on this page:-
and from the collector of that transistor connect it through a 1K resistor to a PWM output of your arduino.