I booked 3 nights Sept 22-25 at Yaang Come Village through Atsiam and my credit card has already been charge the entire amount. Is this normal?

Yes. Most booking sites require payment in advance. If your stay is in the far future, with some sites you can prepay a deposit and the balance later but still before your stay. I prefer to pay at check out so tend not to use the booking sites for this reason.

The issue is that you are less than 30 days from your stay. I made reservations through Sawasdee for YCV months ago, but since my stay isn't until November, I paid one night and will pay the rest in October.

yes it is normal, and these are the terms that are on their site.
they have the cheapest rates for YCV

i find that if the stay is coming up soon then they charge the whole thing immediately....the standard is to charge the remainder of a stay 14 days before the stay commences

At Siam charge the whole lot straight away regardless of how far in the future it is. They do seem to be very good about refunds if you have to cancel. As they can be significantly cheaper for some hotels its not so bad having to pay the lot - if the price is similar I'd prefer to pay later! Have had good experiences with At Siam so far though.

I stick with the R24 sites, pay 1 night deposit, balance a couple of weeks before arrival, even if need to cancel you only have '1 nights money' tied up and not a large amount, suits me fine and never a hitch in years and years, I don't care if it's a few bucks more I don't want to have to pay off those amounts on my card or pay the interest on the balance to the c/card company if not paying straight off. Also the range and choice in my opinion can't be beat. Also like that the booking goes 'direct to the hotel' who respond.

But for nancydart, as others have said, it's so close to time that is why. If you want last minute within 20 days RTG is excellent as you only pay 12% of the bill to secure the room plus I think USD3 fee, that's all you pay, then you pay the balance at the hotel, that is a great deal.