And I'll say no polyester or rayon, cotton only
Poly and rayon dry fast, but they make you sweat, natural fibres breath more, more comfortable, though they do take longer to dry, silk dries quickly but crushes even faster.
Are you just going to tourist areas? You can get away with being a tourist and wearing skimpier clothing that would be classified as immodest elsewhere. If you are venturing beyond tourist areas, the more covered up the better. If you're going to Siem Riep, that means Angkor Wat you need very sensible footwear, lots of walking/climbing, uneven surfaces around the site.
If you are visiting any active temples, you should be pretty conservative in your dress, that applies to all 3 countries.