it might be your 8th entrance or never...nobody every knows but a blunt fact at 3 is very disheartening to everyone and its not verifiable...so in turn its a made up number
Officially you can do 2 land border crossings per calendar year (01 JAN - 31 DEC) and there is no official limit on Fly out/In border bounces, however you may run into an overzealous I/O that sees otherwise...you can also extend all Border Bounces at immigration for 1,900 THB for an additional 30 Days, until the I/O says you can't...you can buy a 60 Day SETV at any Thai embassy/consulate for 1,900 THB and you can extend that at immigration for an additional 30 days for 1,900 THB (90 Total Days in Thailand), you can get 2, 60 Day SETV's back to back but once again you may run into an overzealous I/O that says that says you can't when you arrive back to Thailand...Good Luck, the Royal Thai Embassy and the Royal Police don't communicate well together so the embassy will sell you visas and the I/O's will deny them...