I think the OP has already flipped kiddo in the secondary car and is asking about the primary.
Many posters are ok with turning at 3. I err on the side of waiting to 4+, for a couple of reasons. First and foremost, we know that ossification completes between age 4 & 6 (I have also read 3&6, but read 4&6 most commonly,) and it is that ossification that helps protect the fragile spine. It is a progression, so it starts around age 1 and continues to 4-6, which is why that's the gold standard. Every extra day you can give your child gives them that much more of a chance to mature that spinal column & protect against spinal injury. Also, the NHTSA rec goes through age 4, unless the seat is outgrown before that.
Personally, I am not a fan of making kids less safe "just because," if they are happy RFing. My nanny kid is 3.5 and I've been debating whether I will turn him at 4 - but since he doesn't care how he sits, I'm having a hard time justifying it. Obviously if there is a REASON that is a different story, outgrowing by weight or carsickness or unhappy kiddos, but turning a content kid "just because" is not something I often suggest. Been musing that myself lately.
Plus, you invested in a seat that can allow your child to RF longer than any other, I would take advantage of it