Fitting three across is just a bit of a trial and error process really. You are best off finding a large baby supplies store, or a car seat specialist if you have any locally, and making an appointment to go there and try several seats in different combinations in your car. It takes time so they usually appreciate a phone call first.
Some general tips -
Check your car manual for any restrictions about which types of seat can go in any of the seats.
Isofix seats make things tricky as the position is fixed, whereas when seatbelt fitting you have a tiny bit of wiggle room in positioning.
You probably won't be able to spin your Joie with 3x seats anyway, so likely worth replacing that (unfortunately!!)
Overall width of seats does not tell you as much as physically trying them. It doesn't matter how wide they are at the widest part, if they don't touch each other there. It only matters how wide it is at the parts where they touch.
Car seats are allowed to touch each other, but not push out of place.
The 25kg Swedish type belt fitted ERF seats can be great options for 3 across because you have flexibility in how far forward/back you place them on the car seat, plus they fit such a wide age range (at least 9 months - 6 years, some do from 6 months, some children fit until age 8). Volvo cars are set up perfectly for these and already have the tether loops in place, so super easy! Axkid is the most popular brand, so check the Axkid website for stockists local to you.
It's common that alternating rear/forward facing will get you the best fit, but it is often possible to have 3x RF seats if you prefer that.
The two side seats are not equal, try both combinations. This is partly because they are sometimes slightly different widths, but mostly to do with the seatbelt in the centre.
It's possible to get infant car seats which go onto a belt fitted base. Joie have some, the older Maxi Cosi Cabriofix does this. These are compatible with most major prams. If you don't need the pram compatibility, look at Graco or Hauck. Sometimes lifting the infant seat up on a base gives you the clearance to fit another seat next to it, and means you don't have to lean over other seats to strap in.
Multimac is worth looking at, but bear in mind that it only has a very short lasting rear facing baby capsule, up to 13kg/around 80-85cm. (They say 82cm). This might be a good option if you are happy to FF and your kids are all fairly close in age. It doesn't work well for larger age gaps, and not an option if you're set on ERF. With twins + 2.5yo it could be a good option, especially if your son is on the smaller side/twins are likely to be too. I believe you can only have 3x rear facing seats in the 4-seater model, though. If you wanted the 3-seater, your 2yo will need to be FF.
If you can turn off your passenger airbag, you can rear face in the front which is allegedly just as safe as rear facing in the back.