The most common and most likely answer to this crossword clue is the 7 plus 7 letter word PRIMARY COLOURS
I've seen this crossword clue in The Guardian
P
R
I
M
A
R
Y
C
O
L
O
U
R
S
Buddy explains!
The straight part
We know that "of which there are three" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case of which there are three = primary colours.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.