Is this the narrowest, safest for 3 across?

Hi Kerry, Thank you for your enquiry. It is difficult to be able to give you an answer to your query without seeing the vehicle and knowing the age/size of your children as there are so many variants that may affect space. The best advice we can give is to go to your nearest Baby On The Move store and try the boosters in your vehicle

The narrow booster is the graco and the Dionos

Hope this helps