mmm, so how to balance between max Torque & max BHP RPM zone?

Even though you get max torque around 3000rpm, and the torque starts dropping after that rpm, the torque you get is still higher than any rpm you get on higher gears, little chance you get higher torque than lower gears unless you are using really close ratio gears, which is not your case.

You need to figure out your torque graph for each gear by figuring out the gear ratios & dyno testing, and determine at which rpm the next gear tend to give less dip in torque.

Try starting a car from 0 on 3rd gear instead of 1st, feel any difference? As the speed goes higher high gears can work more efficiently but if you think starting from 3rd gear thing, you can easily tell there's a major difference in "torque" with different gears. Higher gears are just there so you can move up to a higher speed without reving 10k rpm & increase fuel efficiency during higher speed of travelling.