I'm British, but I can answer for the UK and US:
$100 — a hundred dollars or one hundred dollars?
"A hundred dollars" is how I'd say it in speech. "One hundred dollars" is how I'd write it on a cheque.
$201 — two hundred [and?] one dollar[s?]?
In the UK we'd say "two hundred and one dollars". In the US, they might say "two hundred one dollar[s]".
$1500 — fifteen hundred dollars or one thousand five hundred dollars?
"Fifteen hundred dollars" is how I'd say it in speech. The more "proper" way to say it, and the way I'd write it on a cheque is: "One thousand, five hundred dollars" (never "one thousand and five hundred dollars").
$1525 — fifteen twenty-five dollars or [one/a] thousand five hundred twenty-five dollars?
I'd never say "fifteen twenty-five dollars", I'd either say "Fifteen hundred and twenty five dollars", or "one thousand, five hundred and twenty five dollars". Americans might skip the "and".
For a non-English speaking country, say the number fully using "one", for the sake of clarity. In some countries though (such as the Netherlands and Norway), the use of "fifteen hundred" etc. is the same in that language too.
Because in English "twelve thousand" is not interpreted as 12 occurrences of 1000, as in 12 cars, but rather as one occurrence of 12000. Why it's like this, is a matter of speculation.
Languages are different and I guess some languages view 200 as a plural, as two occurrences of 100. English and Dutch do not, and French, as 200_success points out, swings both ways: 200 being deux cents, but 201 being deux cent un.
Best Answer
First off, you're writing a single number (three thousand), so you should not write part of it in numerals and part of it in text - it should be 3,000 or three thousand, never 3 thousand or three 1000.
The number is used as an adjective here, describing the number of dead. Adjectives are not pluralized, so it should be simply "three thousand dead" or "3,000 dead".