What type of visa do I need for a 90-day stay in Thailand with multiple entries?

@Ozan ****

you could enter on a 30 day exemption then go to Cambodia and Vietnam then return to Thailand on another 30 day exemption that can be extended in country for another 30 days if you need it.

Then go to Cambodia, and from there to Vietnam, before going back to Thailand, or start and end the trip by visiting the other countrys

@Stephen *******

read my next comment.

@Ozan ****

with a German passport you'll receive a 30 day stamp every time you enter Thailand (it's called visa exempt).

You state you need multiple entries, so if you'll be leaving Thailand at least once every 30 days then you don't need to worry about a visa at all. Just get Visa exempt each time you return.

If your first trip out of Thailand will be within 31-60 days of your arrival, then you should apply for a tourist visa through the evisa website and that will give you 60 days when you arrive. Then during that time you would leave and return and get another 30 day visa exempt stamp.

Each entry with a tourist visa or visa exempt can also get a single 30-day extension from your immigration office in Thailand.