I was like you, 3 little ones and a husband who worked away a lot. It's really bloody difficult doing it all on your own.
Thing that helped me the most, was having a routine!. Personal hygiene was completely non negotiable, every night I ran a shallow bubble bath and dunked the kids in it. It was part of their wind down bed time routine.
Beds changed on a Saturday morning. You need 2 sets of bedding per bed. I used to keep the fresh set folded up, under their mattress, waiting to go on. That gives you a whole week to launder the dirty ones. 🙂.
School uniform was a priority for me, I didn't want them being ridiculed so I would spend an hour on Sunday, ironing a weeks worth of uniform, so lovely for them to have fresh clothes laid out every morning.
I sorted the kitchen as I was going along so I would never sit down for the evening until the kitchen was sorted, with everything put away and the surfaces wiped down.
Same with the bathroom, keep the cleaning things in the bathroom and never leave the bathroom without folding the towels, wiping the loo, etc. Even if it's just with a spray and a bit of loo roll. "Method" sprays smell gorgeous.
Floors are less of a priority, we have wooden floors so I just swish round with a lightweight rechargeable stick hoover then a speedmop. It takes a few mins!
It's difficult with small kids but clutter is definitely a problem as you waste a lot of time looking for things and it takes up mental headspace. Try and make sure everything has a place, for us it's coats on each person's coat hook. Files for important documents, everything else is shredded as it comes thro the door. Try to get some sort of organisation in your fridge and kitchen cupboards so you can see what you have and what you need to buy.
Wicker basket in kitchen for recycling, this gets carted out to the bins several times a day (your older DC can do this!).
Don't, whatever you do, be tempted to get a pet. They increase the workload immensely and if you are not a natural cleaner, your house will deteriorated very quickly.
Maybe pay for a deep clean to get you started.
Forget the car. 🙂