Maybe removing the or would help?

• return NULL ({-1, -1, -1} for C)
• return NULL if not C, {-1, -1, -1} for C

@Chrono79: thanks!

@S4ule, not, literally it says:

return NULL, or {-1, -1, -1} for C

your read it as:

return NULL or {-1, -1, -1}

The comma is there to separate both cases.

then return NULL, or {-1, -1, -1} for C

Means that generally you return NULL but {-1, -1, -1} for C language. Yes a bit too terse.
BTW you could have seen in "Sample Tests" this test:

int a2[3] = {-1, -1, -1};
dotest(820, 81, 550, a2);

which illustrates the case where "v1 >= v2".

or {-1, -1, -1} for C

... Your language is C? So return {-1, -1, -1}.

Fixed.