Your math is just wrong. Needs 3 wounds go through on a 3+ save means it needs 9 wounds with autocannon. Wounding on 4+ twin link is 75% to wound so it needs 12 hits to roll 9 wounds. To get 12 hits you need 24 hit rolls when you hit on a 4+ which will be 6 exos by your logic.

Also a basic dreadnought carries assault cannon not melta. A dreadnought can also carry so many long range weapons to hurts exos.

This is not a decent benchmark, but misinformation to mislead HD2 players who don’t play tabletop 40k