I've got 3 kids and a Citroen Grand C4 Picasso, which I love!
Another poster has mentioned the Multimac. Please avoid this product! You can only rear face to 13kg - the same as an infant car seat!
The following is taken from a list posted in a group I'm in on Facebook, which may be of use.
#THREE-ACROSS
Cars which take 3 across quite easily -
VW Sharan
VW Touran
Seat Alhambra
Ford S-Max
Ford Galaxy
Citroen C4 Picasso or Spacetourer (5 seater)
Citroen Grand C4 Picasso or Spacetourer (7 seater)
Peugeot 5008 (MPV version)
Peugeot 5008 SUV (2024 model no longer has three full seats)
All of the above have 3 separate, full sized seats with isofix from around 2010 onwards. The first 3 also have isofix in the boot seats from around 2015 onwards. These cars can typically accommodate just about any combination of seats. If you know you need 3 across, these are definitely your best options.
Audi Q7 and Volvo XC90 can both usually take 3 across, though the back bench is a bit narrower than the ones above so you might find some combinations might not work as well in them as they would in others, though still doable. The Audi Q7 has isofix in all seats from 2015 onwards (except the drivers seat obviously). The XC90 only has isofix in the outer two middle row seats.
☆ Cars to Avoid ☆
There are some cars which you need to avoid as it's not possible due to the User Manual not allowing seats in the centre seat...
• Kia Sportage - Mk5 2021+
• Mitsubishi Outlander - also doesn't allow car seats in the boot seats
• Nissan Qashqai - 2nd gen 2013 to 2020
• Nissan X-Trail - 3rd gen, 2013 to 2020. Also does not allow car seats in the boot seats.
• Renault Kadjar
• Renault Scenic/Grand Scenic (2016+)
• Vauxhall Zafira Tourer
• Vauxhall Antara
• Vauxhall Crossland
There are more on this list but I can never remember them all and forget to update when I do! Always check the User Manual of any car you intend to purchase to ensure that the manufacturer allows car seats in the centre seat.
☆ Support Legs ☆
An important thing to take into account is that there are a lot of cars which do not allow Support legs in the centre seat. This is something you should always check in the User Manual of the car you're looking to buy. Most of the ones in my list of 3 across cars do allow them, but it's important to check this first. Some which I know do not allow them:
• Audi Q3
• Audi Q5
• Audi A3
• Audi A4
• Citroen C5 Aircross
• Citroen C4 Cactus
• Mazda 6
• Mercedes CLA
• Mercedes GLB
• Mercedes GLC
• Nissan Qashqai - mk3 2021+
• Nissan X-Trail - mk4, 2021+
• Seat Ateca
• Seat Tarraco
• Skoda Kodiaq
• VW Tiguan and Tiguan Allspace (both MK's)
• VW Toureg - MK3
• VW Passat
• VW Golf