Give it a break for now and come back to it in a couple of days. I feel like these things can swim in your head and it makes it hard to take on new info or organise the info that you have. There will be a solution - people have had triplets before, or twins when they aready have 2+ children, and childminders often have more than 4 children to drive around.
Get on Twins Trust or the multiples board and see what other people do. Just gathering all options so you can write them down and look properly with DH rather than either of you writing things off instantly because you're stressed.
For when you come back:
I'm sure you can use 2x isofix seats in middle row of 5008 and still use the back row. However, I don't think it takes child seats in the rear seats at all. They are only smaller seats.
At least, I hope so - as this is exactly what we plan to do when we get ours! 😬 We have 2 in isofix seats plus an adult sized teenager. He doesn't come with us in the car v often but currently he's stuck between the two toddlers in the 3008 and it's really cramped for him. Once we get the 5008, he can choose middle seat which is full sized or back row.
But anyway - I thought the rule of thumb was just that you can't have 3x permanent fixtures in the middle row (and this applies to any/most 7+ seaters). So 2 there should be fine? Then you would still have to put one baby in the front. 5008 has no isofix in the front (3008 does).
You probably CAN get four rear facing, if you really wanted to - I am pretty sure there are a couple of 4x RF set ups in this album (in fact there is a 5x ERF seats near the end). I'm sure some of these set ups are ignoring the third row access rule - so def check manuals etc before you commit to buying anything. https://www.flickr.com/photos/153076285@N05/sets/72157687963842866/
Have you given any thought to belt fitted infant seat bases? You've got a choice of:
Maxi Cosi original Cabriofix plus Easyfix base
Cybex Aton 5 plus base 2-fix
Joie i-Gemm 3 or i-snug with i-base 2. Or the cheaper Clickfit base for the older Gemm models.
(Off the top of my head. I will look for more, if you like the idea but don't like those seats.)
I know you've already got car seats but if you're looking at buying a new car, then the cost of replacing one of the car seats + base is fairly minimal in the context of that plus it means you can have the base convenience for all 3, without necessarily needing isofix for all 3.
Everyone with a big family on the FB pages raves about the Alhambra.
Something with 6 seats facing each other is also a good option - your eldest's Kidfix can go on the rearfacing seats, and the triplets can go on the forward facing row in rear facing seats. Once the triplets get to high backed booster age you can put them into 3x Britax or Cybex boosters on the rear facing seats, and your eldest will be much older then so can sit in the forward facing row facing them.