I think you'll get the same feeling in the 3. But all indications are way more knee/head room than you'd expect. Not sure which one I'd rather have, but I also never sit in the back up my own car, and my kids are still in car seats.

I haven’t spent too much time back there since I have two little ones (in car seats) back there.

But I did sit back there when we first got it and was really surprised at how much head room there was. I think the extra head space might be due to the glass roof.

The low floor gives the opportunity to have more headroom in the back. Yes, if you're used to sitting in front seats, it's lower than you're used to.