How Do You Write Three Thousand Four Hundred Thousandths In Decimals?

3.400
 
One-thousand (1,000) thousandths is equal to one (1), so three-thousand (3,000) thousandths is equal to 3, which is a whole number and is on the left side of the decimal.  Four-hundred (400) thousandths is a fraction that is also equal to forty (40) hundredths or four (4) tenths, so the four is on the right side of the decimal.  The two extra zeros show that it should be read as 400 thousandths rather than 4 tenths.
 
If you had asked how to write three thousand AND four-hundred thousandths, the answer would be 3,000.400.  The decimal is read as "and".

Using the digits 0, 4, 7, and 9, write the greatest decimal possible, in thousandths>

Given that the word and (which typically represents a decimal point) never appears in the question, so I take the number to be 3,400 times 1/100,000.  Since 1/100,000 is .00001, the complete figure would be .03400.  Since I somehow doubt that the measurement was that precise, I would simplify it to .034.  If it is a DNA test, though, then back to .03400.

Four-hundred%20thousandth

3.400

34000

6.004

3,000.400

2,300,001.14

300.400