Answer
250 words
Work Step by Step
Represent the unknown:
Let
$x$=the number of words he has written
Since he still has to write three times as many words as he has written, then he still has to write $\color{blue}{\bf{3x}}$ words.
The total number of words is 1000, thus,
$x+ 3x = 1000
\\4x=1000
\\\frac{4x}{4}=\frac{1000}{4}
\\x=250$
Therefore, he has written 250 words already.