Additive divisor problems

Question

There is plenty of literature on the binary additive divisor problem, that's evaluating \[ \sum _{n\leq x}d(n)d(n+h)\] for various $h$. Why is there nothing (or I don't find) on \[ \sum _{n\leq x}d(n)d(n+h)d(n+h')?\] Is it too hard, too uninteresting (as in, offers nothing new after the binary case), or something else?