To get 3 in a row it can help a lot to have alternating directions e.g. forward facing toddler in middle seat, 2x baby carriers at sides. With twins isofix bases would probably be preferred for ease, but you will find you get more room to fit the toddler seat in if you belt the infant carriers. If your toddler's current seat will fit in the middle, try fitting it there and then measuring each side seat. Often the middle is slightly off-centre, so you'll have more room on one side than the other. If you already have one infant carrier from your toddler, you can then evaluate whether it will safely fit on either side, and how narrow a seat you'll need for the other side.
Another option might be to look into permanently installed rear facing seats for the twins such as the Joie Stages. You wouldn't be able to carry the babies around in their infant carriers, but there is less danger that you will accidentally unbuckle the toddler seat when trying to remove one of the babies' seats. You would probably need a slimmer seat for the toddler in this case.
If you do need to look into slimmer Group 1 seats for the toddler, look at:
Britax:
Eclipse
King
First Class (Can also rear face up to 13kg only)
Duo.
Maxi Cosi: Tobi or Axiss (you lose the turning function unfortunately, but you might get that use back again later!)
Joie: Tilt or Steadi (both can go rear facing to 18kg too :))
Graco Slimfit (can also be rear facing to 18kg)
These are all seats with good safety records that are 45cm wide or less. The two Joie ones are the slimmest. However overall width is less important than the shape of the seat bases.
You could also consider getting your 2yo a larger (25kg) rear facing seat and turning him back around. Many children find this a great adventure, and it's much safer for them to travel that way. The advantage of these seats in this case is that they can be installed much further forwards on the car bench seat, allowing for more space for an infant seat base if that is an option that you want.
If you are going to get a seat which has the capability to rear face for your toddler, I would recommend buying that one at least ASAP and putting him back rear facing even if you have to turn him round when the twins are born.
As with many things though, it is often a case of trial and error! The best thing to do ideally would be to look for a car seat specialist local to you who can spend some time taking seats in and out of the car and seeing what will fit exactly. It's also important to check your vehicle manual and see if they even permit a child seat to be fitted in the centre, as some do not.
Car seat specialists: www.carseatadvice-uk.com/specialist-retailers.html