I wonder if 3-thousand dead can be rewritten as 3 thousands dead? Are both of them correct? What's the difference between them?
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".