Things change. Used to be you didn't need a neutral in a light switch. Now you do.

Used to be you didn't need a neutral in a dryer plug. Now you do.

Used to be you didn't need bond wire in emt. Now you do.

Code changes. Constantly.

The point is that cutting wires flush to the box doesn't help anything, it just means now there's a hindrance in the future. Maybe code changes in the future. Maybe a future person living in the house wants to do something else with that wire, but now they can't, bc an important wire is snipped.

Don't downsize if you don't have to. Every splice is a place where it can fail, whenever possible just run whole lengths.

Don't cut the third wire, just cap it off and tuck it into the back of the box. As an electrician, I fucking hate seeing wires chopped close to the box. In ten years or whatever, maybe codes have changed and you'll need a neutral, but now there's not enough to work with and it becomes a major pain in the ass.