Here is a cheaty way to do it:
You can count the total number of ways 3 points get selected out of the 6
\({6 \choose 3}=20\) (Order does not matter)
Now, there are some cases that cannot be counted (straight lines)
DQC and APB are straight therefore are not triangles.
So 20 - 2 = 18? Counting and probability are my weakness I'm not sure if this is right.