Well, Dale, Texas is a BIG state… The advice that you mention was meant for people who need to use 2 deeps for their hives (I think that was specifically for a hive in NorCal). In southern California, most of us hobby beekeepers use 2 deeps. Commercial beekeepers use one deep, but they manage their bees very differently on the whole. Subtropical climates - like the area where the Flow hive was developed, may only use one brood box. I know that @JeffH uses just one brood box. The advantage is that it simplifies inspections tremendously. The disadvantage is that if you have a nectar dearth, you have to monitor closely and consider feeding sooner rather than later.
If most of your locals use 2 deeps, I would too. If they don’t, then don’t do it. I add deeps underneath because it causes less disruption to the queen’s laying pattern. But I only do it when I am adding a second deep for the first time on a particular hive. Once it is established, I add honey supers on top.
If your area recommends 2 deeps, but you only have one and 2 mediums, then you have 2 choices. The first is to continue with what you did last year - you successfully overwintered a colony, and that counts for a lot. I would still make sure they have space in the brood box for the spring buildup, it can be amazing how fast they fill it. The second choice is to put a second deep below your existing brood box, remove one medium and wait for them to use the bottom deep. They will need a lot of that syrup to draw out the lower deep.
If I was in your situation, I would stay with one box and 2 mediums. You know how to make that work. Unless there is a good reason to change it, I wouldn’t.
You should never add an empty box unless all other boxes are very full. So if you have a full brood box, then a new brood box below, you should not add the Flow super until both of the lower boxes are 80% full of brood, pollen or honey, with all of the comb fully drawn. If you do add too early, the bees will not use it, and they may have trouble defending it from pests and robbers. Absolutely, they will put energy into filling up the new deep. If you want a harvest, that will be a problem. But if they need the stores to survive the winter later, that can be a problem too. That is why beekeeping is an art.