No advice on the Nissan, but the Corrola I can help with.
I doubt 3 RNs will work. First off, if you put a rfing RN Outboard, you better have awfully short people to go in the front seat since the rfing boot for the RN makes it impossible to install much more upright than 45 degrees. Other convertibles can be installed as upright as 30 degrees for older kids. Outboard ffing, the RN is a PITA. It is possible, but there are lots of easier installed seats that will work. However, the RN fits in the center both rfing and ffing with very little effort.
Here's what I would do. 4 yo, ffing Radian center and twins in infant seats outboard. The Graco Safeseat1 would be best if it fits because you'll get the longest use out of them. This will allow the 4 yo (who has the least amount of side impact protection just because of being ffing) have the safest center seat while still allowing her to climb in easily over the infant seat base.
Once the infant seats are outgrown (hint, if one twin outgrows it before the other, only switch that one so 4 yo can stay in the safer spot as long as possible.) buy a convertible with really good side impact protection and a 65# + ffing weight limit for the two outboard positions. (you'll have to try out different seats to find what will work. Hopefully you'll get at least 18 months out of the Safeseats and new convertibles will be out to give you even more options.) When both twins have outgrown the infant seats, turn the RN rfing in the center for the smallest twin, put one of the new convertibles rfing outboard for the bigger twin and use the other one ffing for the 4yo. You can always sit on the ffing convertible to buckle up the twin in the middle position. Once the twins have reached the weight limit for rfing, 33-35# depending on the seat, and every body has to go ffing, put the smallest child in the center.
Basically, 3 in a row is about just getting in and trying things to see what will work. I'm confident you can get the RN in the center with infant seats outboard though, so that should buy you some time.
Edited because I just realized I used the percent sign in place of degrees. *smacks self in head*