How to do more damage with limited stardust?

Option 1 would be the least preferred because of the diminshing returns that holds in Pokemon GO.

DBS lover it seems ?

Powering up the lower level Machamps is a better immediate investment than powering up the higher ones, but powering the higher level ones is better in the future if/when you get better Machamps.

If you power up the first Machamp up to level 36 it'll gain 1 damage to Counter and 4 more points of HP for 50k dust and 58 candies.

If you power up a level 20 Machamp to level 29, that'll cost 65k dust and 58 candies, increase it's HP by 23 and increase Counter's damage by 3 points, with 14 or 15 attack IV.

You get far more gains from powering up the lower level Machamps,unless you already have 6 powered up Machamps that you can't bring your newly powered one into battle, or raids are finished so quick that you don't get to use all 6 of your Pokemon - although if that is the case I would suggest making smaller groups so that everyone can do more damage, get more balls and therefore have better rewards and a higher chance to catch the Pokemon.

Definitely #3, power up Trunks and Gohan. If you're in doubt or need to convince yourself, run the Pokebattler sims. Two level 30 mons always do much better than one level 30+ and one medium-level, given the same total stardust investment.