On entering Thailand from Tachilek my triple entry Tourist Visa was stamped "USED" even though I have only made 2 entries. Didn't notice until I got back to Chiang Mai. The question is, should I sort this out with Immigration in Chiang Mai or just wait for my next crossing?

Thanks in advance...

Try CM, but they might send you back to the border to do it.

Possibly true.

But before you go asking are you absolutely sure your triple entry visa hasn't expired,?

There should be a stamped entry that says something similar to "must be used before .... stamped date ...."

You must use ALL THREE entries before that date.

That terminology confuses a lot of people but that's what it means.

So make sure that date is not passed,

If you only used two entries, but you are past that "must be used before" date, your 3rd entry is invalid as the visa has expired on that date.

Not sure if that's your case or not, but you should check to be sure.

Triple entry visas are often issued with an expiration date of only 180 days from their date of issue... so you have to watch carefully that expiration date .... because as I said all 3 entries must be used before that expiration date.

just wait for next time, make sure you go to border before the Visa validity has expired.