It's been mentioned a time or two before, but there really is not a clear consensus on it...
If you have enough torque in your motor, you can start out in racing gear and go..
If your motor is a little weaker, or your running tall gears, you may have to "slip" the clutch as you take off in high gear.
If your motor does not produce enough torque to pull you from a dead stop ... get a bigger motor....:evilgrin: Just kidding
You can start out in a "partially" lower gear , you mention 3rd and going to 5th... lot of guys in IMOW o just exactly that...
Most prepared class racers will run in 3rd or 4th, (depending) and will start out in 2nd or 3rd (respectively)...
Yes hit the clutch to take off pressure, hit high and release... no more shifting needed...
Under no circumstances should you shift from 1st thru gears to get to high (one shift only) Nor should you need to "downshift" thru the corners and up shift on the straight..
If you feel you need to do that, you most likely do not have the correct "final ratio" and should be adjusting pulleys and sprockets to find the "sweet spot" in the torque curve for your engine.