If the front seat is already occupied, one option is for the other adult to go in the back on one side, and put two children in the middle and side seats. This is usually easier and more comfortable than trying to fit an adult into the centre.
It is safer to have a rear facing child in the front if you can disable the airbag. A forward facing child in the front is at risk from the airbag as well, so check your car manual to see if this is allowed.
I do not think you will get a rotating base in which is usable without getting a larger car. The problem is when you rotate the seat, it sticks out backwards which doesn't work when you have a third seat in the middle. They also only work with isofix, so you won't be able to put one in the front seat.
The good news is though you don't need a rotating base for a baby seat anyway, place the seat down in the boot to get the baby in with the seat facing you, and then transfer it onto the base or place baby seat on the vehicle seat facing whichever way you like and then turn it backwards to strap in with seatbelt.
When you move to the next stage seat, look at the extended rear facing ones which use seatbelt and tethers to fit, they have low sides so are easy to use without rotating. They also tend to be easier to fit three across.
You can sometimes fit three seats across in an ordinary car with a 3/4 sized middle seat. The key to this is basically being willing to try multiple combinations - local independent baby stores will often do this or a larger chain like John Lewis. You will want to check the car manual to see if a seat is even allowed in the middle seat. If it is, check whether a support leg is allowed. If it is, you can likely fit an extended rear facing seat there or a belt fitted infant carrier base with leg. This can sometimes lift the infant carrier up high enough to clear the lower edge of a booster seat. Consider trying 5yo in an extended rear facing seat for a year or so until the baby is ready to move into it. They can be easier to get 3 across because the tethers give you flexibility in terms of front to back positioning. The actual widths of the seats matters much less than the shapes of them.
If you're struggling to fit three across, it is legal to have the eldest child in the middle seat with no booster as long as they are over 3. However at 6 it's unlikely they'll get a very good belt fit, so probably not a great solution.
You might also consider Multimac - it would be a reasonable solution given your older two children are already old enough to forward face. You could buy it second hand and then resell when the baby has outgrown the rear facing seat, if you don't want them forward facing at 84cm. This might be a pricey solution but could be cheaper than a new car and if you did resell, you could make the money back again.
Lastly, this list is very out of date, but could give you a starting point for suggestions, and proves the point about the car not necessarily needing 3x full sized seats: https://erfmission.com/car-seats-3-across-in-the-backseat/