Trying to think of explanations for why there seemingly isn't the obvious, i.e. a missing person report for three people, at least of one of whom whose '83 Oldsmobile 88 is also missing.
It looks like currently in Florida, vehicles can be renewed for one to two years, and up to three months before the expiration, which is midnight of the first owner's birthdate. So assuming these policies were the same in 2005, hypothetically the car could have been registered anytime in 2005 and remained so until sometime in 2007. If the car still had valid registration at the time it went in the lake, that's a timespan of 2005-2007.
Other theories (in addition to everything suggested in the posts above):
1)The car was stolen, and nobody knew the three individuals were even riding in it together at the time, so none of them were reported "missing with a vehicle."
2) Or 1) and the trio's relatives were afraid to report them missing because they knew about the stolen car and hoped the trio would return in it, but feared this as the saddest ending.
3) The registration expired and to avoid being pulled over, the car was driven only at night, which sadly made the accident more likely.
4) Maybe the car changed hands, whether due to an inheritance (it does look like what my kids would affectionately call a "G'ma/G'pa car") or a sale where paperwork was never legally done. Or it sat for years before someone got it running again and they were out driving it before they had reregistered it. Thus it was never associated with the trio.
5) The car was registered to its driver, and he/she picked up the other two unexpectedly, maybe for a spur-of-the-moment shopping trip at the mall. They may have one or all been reported missing, but nobody knew they were together. Or the car was being used as an unregistered taxi service.
6) They were day labor workers, or in this case, night shift labor workers. Three people were sent to help clean at the mall overnight and never showed. This just snapped into my mind because as a former day labor worker, I often drove my coworkers to job sites and if we hadn't showed, it was literally one-day labor with no obigation to show up the next day. There were people who just stopped showing up, hopefully because they'd found a more permanent job, and neither the job site or the day labor office would have been too concerned.
The location alone, the biggest single-story outlet mall in the US, makes me think this would be a risky place to hide a car with victims. Even at night, you'd think a place that big would have cleaners, deliveries, etc. This makes me think it was more likely to be an accident. I hope their families get closure very soon.