Needing 3 across changes everything. You need to look at the big picture and choose a seat that will work in your 3 across. A Britax might or might not work, a NextFit might or might not work, and any number of other good convertibles might or might not work. What is the year/make/model of your car? If you're lucky, someone here might have experience with it and be able to help you figure out what combinations of seats will fit. Otherwise, your best bet might be going to a store with a large selection of seats (like Babies R Us or Buy Buy Baby) and trying things in your car. If you already have an infant seat and a booster that you would like to use, bring those along. Try a variety of convertibles in a variety of set-ups and see what fits. Once you find a combination that you like that seems to fit, then go ahead and install all of the seats (for the booster, check that there is room for it to be buckled) and make sure that they can be installed correctly when you're using all of them. (This is usually easier if you use seatbelts rather than LATCH to install.) You want the seats to be independently tight, which means that if you remove one seat, then the other is still tight.
I assume you do already have a booster for the 6 year old, do you have an infant seat that you plan to use? What are the seats?