I want to visit Thailand { Koh Chang ) frequently this year.

If I use a Tourist 60 day visa with multiple entries ( 3 ). Does that mean I can visit Thailand, return to the US and re-visit say 2 months later on the same visa using one of the unused multiple entries?

Or does the visa just exist for the 60 days and I must use the multiple entries within that 60 days?

It means you have 3 entries into Thailand, each time you can stay for up to 60 days (which can be extended by another 30 days). But a visa has an expiry date, you must enter all three times before the visa expires. Normally that is 6 months from issuance.

How long will you be staying each time? Without a visa entering Thailand by air you would get 30 days each time you enter.

When you get the tourist visa, 1-3 entries, not 'multiple', the visa will be good for either 3 months or 6 months.

Each of the entries must be used before that date, or you lose it/them.

Each 60 day tourist visa entry can be extended at an immigration office for an additional 30 days.