If you stay in Thailand for 3 months you get 60day evisa and exstend 30days at imigration. If you plan on leaving make sure you book exit flights to show on entry and dont over stay 30days at a time. Common mistake is not all months are 30 days.
Its states onward travel this is proof you leaveing within the visa time it does not have to be a return flight. A flight or bus ticket out Thailand will count. I try book somthing I can use or move.