Hi,
I would keep the HBB for both your older two, it's safer than a backless one.
Have you tried your Group 1 seat in the car alongside the HBB? I'm assuming you have a decent sized car, but you'd just need to check that the two HBB will fit next to each other. My DD1 has a Britax KidFix HBB, it gets quite wide towards the top of the backrest so I know I could not put two of them next to each other in the back because I can see that the one we have overlaps the car's seat.
I have DS (aged 8 1/2 and 133cm tall) on a backless booster in the middle, next to DD2 in her Britax Multi Tech rear facing and DD1 on the HBB on the other side of him. DS can easily manage his own seatbelt as it's easy to reach the buckles, and although his seatbelt buckle is next to the one for the Multi Tech he's never undone the wrong one.
I guess you don't really want your youngest in the middle in a Group 1 seat because you're going to have to be reaching over to fasten the harness each time. It's also tricky to lift a sleeping child out from the middle seat! But not impossible, sometimes on long journeys I put DD2 in the middle to split the other two up!
If you have the Group 1 seat behind the driver it means that your older two can get out of the car safely onto the pavement rather than into the road, but on the other hand it means you'll have to stand in the road while you fasten your youngest in. I have the Multi tech behind the passenger, it can't go behind the driver (me 99.9% of the time, and I have long legs!) due to lack of legroom for me, and so my older two have to get out onto the road. But I have the child locks on, so they can't get put until I've checked it's safe and opened the door for them, and it's not often we park next to the road anyway (we have a drive at home).
And here also, I'd say over half of the kids is DS's class (he's Y3) have ditched the car seats altogether. DS is taller than most of them, so they can't be 135cm tall yet. A lot of them pull a face if I give them a lift somewhere and make them sit on a HBB (DS's seat is the only backless booster I have, in addition to DD1's KidFix I also have two Britax Adventure HBB that I use for spares), but my reply is 'my car, my rules!' And also, as the driver I am responsible for anyone under 14 and making sure they wear a seatbelt/are in a suitable child seat.