The lady in the shop is repeating some myths.
I have looked at pictures of the back seat of your car and I reckon you could fit three across there if you find the right combination, which will probably mean ditching one or both seats you currently have (sorry, but will be cheaper than changing the car, they could possibly be of use to a grandparent?) The Joie 360 in particular is very tricky to use without being able to spin it, and it won't be able to spin when installed next to another seat.
Airbags which are deactivated can't go off, or at least I am assured there's never been a recorded case of it happening. The general wisdom is that the oldest/tallest child should go in the front, but anyone who knows about ERF knows that you can indeed put a rear facing child in the front as long as the airbag can be disabled, and it's much safer than putting a forward facing child in the front. Some car manuals suggest deactivating the airbag for any child under 12.
This is a blog (sorry it is a bit snarky - that is the general tone of the author) which explains why the front seat is OK for a rear facing seat. I probably wouldn't put your baby in it, because your Joie i-Level can't be fitted without isofix which means you'll need a new infant carrier. If you are concerned this blog post is too old, look at the instagram account of this website as they still post regular fit photos of children in RF seats in the front.
https://www.carseat.se/2010/05/28/are-car-seats-in-front-seat-safe/
The main issue with deactivating an airbag is if you're doing this ad hoc, because chances are quite high you'll forget to do it and/or forget to turn it back on when you have an adult passenger in the front and either mistake could have deadly consequences. But this is less likely if you have a seat permanently installed there like one of the Swedish type ones, because installing/uninstalling it is more of an event so you'd be more likely to remember to change the airbag setting.
Multimac's rear facing seat only allows use up to 83cm/13kg and their supposed ERF module has been "coming soon" for over a decade at this point so rule that one out.
If you want to try for three seats in the back, I would find your nearest ERF specialist by looking at Axkid's stockist list and see if they will let you book an appointment to try multiple combinations. It takes a while because you have to try each seat in each position to see how the belt etc aligns and the seats fit against each other. Take your current seats, and any spares you have for travel with you. You will probably need at least one Swedish type ERF seat with tethers, either an Axkid, Britax or Avionaut. Your 5yo or 3yo could use one of these seats now.
The old Joie Tilt is also very narrow and can fit well in a centre seating position, though is only stocked by Kiddies Kingdom and I am not sure how old their stock is. It also is outgrown earlier than other RF seats, but this doesn't matter so much when you have children close in age because you will be more able to hand the larger ERF seat down and it just buys you an extra 6-12 months compared to an infant carrier. I think the newer R129 Graco Extend/Joie Steadi is slightly wider (and MNers report also outgrown before the 105cm limit is reached) but could be worth a try, Halfords Impala might also work in a centre position. Cozy n Safe Fitzroy (NOT i-size) is another option which is similar to the Joie Tilt. RF limit is only 13kg but again you might get to about age 2/2.5 and by this time your middle child will be old enough to vacate the ERF seat.
For infant carriers - it's tricky, and you might want to stick with the i-Level. If you have the LX base rather than the Encore base this might be easier so it may be worth considering swapping just the base. Ideally, an infant carrier with belt fitted base is a great option but they are really hard to find now. You can of course belt fit most infant carriers without the base, but then that adds extra time and annoyance when getting 3 children in and out of the car, and I find most infant carriers are not well designed for belt fitting these days. Britax are good, Maxi Cosi tend to be good and Avionaut Cosmo (but not so much Pixel) are easy to fit with belt. With Cosmo or Maxi Cosi the inserts are so good that you might not miss the recline.
You could look at Joie Sprint as it is still isofix for the speed factor, but might be just that tiny bit slimmer due to lack of base. OTOH the base raising the infant seat up can be incredibly helpful when trying to fit next to another seat, because the handles sticking out on the infant seat tend to be a problem and interfere with the seat next to it. And Joie Sprint is a very unremarkable seat in terms of features otherwise.
For any position where you have 2x buckles next to each other and one is a permanently installed seat like Axkid but one is buckled every time, consider a buckle guard with key unlock for the permanent seat. It helps prevent accidental undoing of that seatbelt.
This post is very dated now but might give you some idea of combinations or models, and can see how 3 seats fit in cars which don't have three "full" seats in the back.
https://erfmission.com/car-seats-3-across-in-the-backseat/