What type of visa do I need for a 2-3 month trip to Thailand as a Greek citizen?

As a Greek citizen visiting Thailand for 2-3 months, you won't need a visa if you plan to enter and exit the country periodically. You can receive a 30-day entry stamp each time you arrive without a visa. However, be aware that if you enter Thailand through a land border, you can only do so twice per calendar year under the visa exemption rule. It's advisable to have proof of onward travel, such as a flight booking to your next destination outside Thailand, to avoid any issues with airlines.