If I extend the tourist visa an extra 30 days.  Than go to Cambodia and back into Thailand twice for two more extensions.

 

Will they ask for onward travel at some point?  Can I buy a plane ticket anytime I am there or do I have to have return ticket?

You don't get extensions by leaving and re-entering the country. You can only get a new entry from a valid visa or obtain a new visa exempt entry.

If you have a valid visa for entry to the country a return or onward ticket is not required.

If entering to get a visa exempt entry you may be asked for ticket out of the country within 30 days to board your flight.

cool

Is your recent stay in Thailand the only stay here in the past year or so?  I ask because when attempting to enter on a Visa Exempt entry, you may receive scrutiny and even rejection, it you have spent significant time here and/or have multiple visa-exempt entries in their system from previous visits. 

 

There is no "set of rules" to know when you have crossed the line, so it is variable what will happen.  I would avoid Poipet - especially for the 2nd attempt at Visa Exempt, if that is what you are planning. 

 

OTOH, if you get a new Tourist Visa at a nearby consulate, you should have no trouble at all coming back in repeated times.   It's the Visa Exempt entries that could be a problem, dependent on your "time in Thailand" history.

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Than go to Cambodia and back into Thailand twice for two more extensions.

 

You can't extend visas inside Thailand or by going to Cambodia.

 

You can get one 30 day extension of your permission to stay at an immigration office in Thailand. You can go to a neighboring country to apply for a new tourist visa or, depending on your nationality, you may be able to get a new 15 or 30 day entry to Thailand on a visa exempt entry by exiting Thailand and returning. The new tourist visa entry or visa exempt entry would also allow you to get a 30 day extension of stay at an immigrations office in Thailand.