I have a double nationality (with two passports). Have been told an 3 entry O visa would be the best way (by Thai embassy that doesn't know I have double nationality). Getting married (using my UK passport) in December. Work of some sorts most likely in future. Anybody been in my shoes, and know how to make the best of the situation? Is there any point getting O visas on both passports? All advice well received.

There is no 3 entry non-o visa only single or multiple entries. The chances of getting a non-o without being married are small.

Which passport you use makes no difference for visas or extensions of stay. For getting an affirmation of permit to marry it could make a difference.

If you can meet the financial requirements of 400k baht or 40k baht income you can get an extension of stay based upon marriage. If not you can get a multiple entry non-o at Savannakhet Laos with no financial proof. Both would allow you to get a work permit if you find work.

Actually there do seem to be several small Consulates that will issue 2 or 3 entry non immigrant O visas (have seen reports from both Oz and Europe) but they are not normal or cost effective for most. If you are getting married on the UK passport I would get your visa on the same.

The only one I know of is Melbourn that only does two entries.