Depending on your car seats, yes it will fit 3 car seats across. I had a Britax Advocate Clicktight rear facing and two Diono Radian 3RXTs forward facing. Like
@JLOC said, I used the car seatbelt for the clicktight seat because it's built to use that. My kids were too heavy to use the isofix/latch mounting points for the forward facing seats. If you are using an infant carrier with a base, the isofix/latch would work well.
The problem I ran into when my oldest moved into a booster seat was that the base of those seats were larger than the car seats, so now I had to switch to two Diono Radians and one booster seat and it is really tight in there. Luckily we have a larger vehicle we use for most family outings.