so you are seeking to get the SETV single entry tourist visa as a way to stay in Thailand for another 90 days (60+30) ? Back to back entries into Thailand, either by Visa Exempt Entry or with a SETV single entry tourist visa may cause you to be questioned by Thai Immigration Officers upon arrival.
Be prepared to answer Why you want to stay in Thailand? You're not a tourist.
Have a booking for your initial week or so in Thailand, show ticket departing Thailand within the 59 days of your arrival into Thailand, and 20,000 Thai baht in CASH if asked by the Thai Immigration Officer who greets you on arrival.
Were there any other recent history of visits to Thailand other than the 3 months stay you just completed?
you are eligible for the Visa Exempt Entry into Thailand. Gets you 60 days STAMP on arrival Plus one 30 day extension of stay at your local Thai Immigration office for 1,900 Thai baht.