Pull a single entry tourist visa from a thai consulate BEFORE you wing your way here.
You'll get stamped in for 59 days when you arrive it's really 60 days, BUT the day you arrive (no matter what time) is day ONE.
Then go to the immigration office and get a 30 day extension for 1900baht. <-They add 30 days onto your existing admitted until stamp.
That's as close to 90 as you're gonna get.
The reason I suggest you get a tourist visa BEFORE you come here is; since Dec 31st, 2016 they passed a rule were you can only "border-bounce" for a 30 day visa exempt stamp by a land border crossing TWO times in a calendar year.
Getting the tourist visa buys you as close to 90 days as you can get (stamped in for 60 + a 30 day extension at the immigration office for 1900baht)
PLUS if you needed to you could apply for another extension (which would be denied) BUT they'd sell you the "extension denied" 7 days to get outta dodge stamp for another 1900baht