a diesel drill must either drill 3 by 3 or even more, otherwise I don’t see the point in switching to it, a steam drill drills much more efficiently

Well the diesel drill has a higher mining speed compared to the steam drill for single blocks, at least if you use sufficiently powerful fuels such as Boosted Diesel.

Making it 3x3 or more would be pointless since you don't need to mine if you get to the diesel drill, IMO.

this is minecraft, I need something to make long tunnels underground, 3x3 and not 1x2, otherwise I would just take a netherite pickaxe with mending

Fully agree.
I thought I am doing something wrong for 15 minutes until I reached this and figured out that it is actually does not have 3x3 digging mode 🥹

don't need to mine if you get to the diesel drill

There is a lot of purposes, for example, terraforming, cleaning up territory for buildings.