I done 54 days. Left for 12 days then came back and they questioned me. Asked to see tickets out the country. Asked where I was going and staying etc. I think the 3rd entry would be the problem.
When arriving in bkk airport. Just after collecting your bags or before. Goto a mobile stand and give them your phone. Choose your tariff and let them sort it for you. Takes about 3 minutes and costs about 1000 baht for 30 days