Krabi vs Koh Samui vs Phuket
 

Hi

We are planning to spend 3 days at a beach destination in Sept and can't make up our minds whether to visit Krabi or Koh Samui or Phuket.

I understand that it's the rainy season in parts of Thailand at this time but don't want to visit somewhere too touristy .. just a few days of relaxation with a bit of sightseeing.

Thanks for your ideas....Freddo

All of those places have very touristy areas and quieter areas, so none of those places is inherently better than another. If you opt for a quieter area, it will be a longer trip to a town where you'll have a selection of bars and restaurants. There are trade-offs for everything.

What kind of sightseeing or activities are you looking for? That may be the deciding factor for you.

In September Koh Samui, Koh Tao and Koh Phangan would be better weather than Phuket, Lrabi, Koh Lanta area.

Koh Tao is mainly popular for diving, Koh Phangan although famous for Full Moon Partiers is relaxing outside of that period and some nice place opend up now in the higher end.
If you want to add 'sightseeing' not that much on smaller islands like Koh Tao or Koh Phangan, however you have the opportunity to go between them ( plus Koh Samui ) very easily.

The thing is you only have 3 days, so you don't want to waste time traveling, so that may put the extra boat trip necessary for Koh Phangan or Koh Tao out of the picture.
Perhaps a quiet resort on Koh Samui might suit.

If beach quality is not too high on the list perhaps some mainland beach area might suit you.
If you really just want relaxation on the coast with a little sightseeing perhaps somewhere like Evason or Aleenta Pranburi, 3 hrs/3 hrs 30 by limo or car from Bangkok, no flight necessary.

Another option if you want something 'special' might be the Paradee on Koh Samet, that's a few hours by road from Bangkok and then a short boat ride, smallish luxury resort, pool villas etc, not 'so' cheap but not too expensive, depends on your budget.