HiMy first gear ratio is about 1:10 and my 4th gear is about 1:3 (opel corsa 88').
Does that mean that in first gear I could have 3HP X 10 = 30hp?
You can't have more HP with ratio. Only torque change when you change ratio.
3HP @ 1000 rpm = 15.76 lbs-ft of torque.
3HP @ 100 rpm = 157.6 lbs-ft.