Three Red Seat Indicators and Beeps while driving

If you purchased this car from a dealership and they gave you some sort of warranty - have them fix it.
If not, you can simply fasten all the seat-belts in the rear to prevent this from happening. The other option is to have someone connect it to a diagnostic tool and disable the lights and alarm; unfortunately that's going to cost some money. Unless someone here, with a diagnostic tool will volunteer to help you out.

Hope this helps....

 

Unless you have more info, and based on what you have posted on this matter AND based on my Gen 4 experience and Owners Manual, you don’t have a problem. I believe what you’re experiencing on this thread is the PriusChat “Chicken Little” syndrome: First indication is for ANY problem (real or imagined) you will get the 12 volt battery is the problem (and age of the battery is not usually known at that point). Second indication is you will get advice that will range from “blown head gaskets” to “you need a new hybrid battery” and everything in between.
If this is your first hybrid I would recommend checking out John Kelly’s videos from Weber University specifically for your Generation (seems many here are unaware of the difference between the Generations); and check out the Car Care Nut on YouTube; AND MOST IMPORTANTLY if you don’t have a manual for your specific year get one (you can download one from Toyota. While you’re there get the Maintenance Schedule and pay attention to the foot notes in it.
FWIW the rear seat belt lights DON’T light up individually in my ‘22 and they stay on for what feels like forever sometimes.