No.  You cannot go by the fact that the word "Eight" comes before
"three times a number" in the English sentence.

You ask and answer these questions:

Qustion:  What is it that is being made less by eight?

Answer: "three times a number" or 3x is what is being made
         less by eight.

Question: How do we make 3x less by eight?

Answer: By subtracting 8 from it.

Question:  How do we do that?

Answer: By putting 3x down and then subtracting 8 from it, like this:

        3x - 8.

So always ask and answer the above questions.  NEVER just go by the
order in which the 8 and the 3x come in the English sentence.

To make something less, write it down, and then subtract from it.

Edwin