D&D isn't designed for this. Of course if you play a game against the way it is designed you're going to run into problems.That's 1.5 combats a session.
That actually feels about average to me for most D&D campaign's I've played over the years.
Shockingly, I've played D&D games where sometimes a whole session goes by and there's no combat at all!
The problem here isn't the resting mechanics.
If you're having so few combats then why do the PCs need a long rest?If you're getting in about 2 combats a session than I suspect you're not going to be waiting 6 combats before allowing PCs a long rest.
You have to factor in real world time considerations as well.
Why be concerned about the ability to cast Fireball when there is nothing to Fireball?