you are correct only 3 months but if you are in Thailand they don’t care if you do extension at immigration & boarder run, only issues will be when you leave & try to leave Thailand & re enter on a flight. That’s why we got our DTV as want to stay long term & have a Thai bank account & can support ourselves.
yes last time we were turned away as extended visa by 4 months ie 60 days on arrival 1 immigration extension of 30 days & 1 extension of 30 at immigration giving us 4 months. We were told only aloud 3 months! All good now as have DTV visa if your not leaving Thailand it’s not a problem but as soon as you leave & try to return via Flight it will flag up at the airport.
only 2 boarder runs per year now & Fly in two times per year if you leave Thailand & have done more than this & try to get back into Thailand via air Immigration at Airport will not let you re enter.