Does anyone understand this?

Mahler wrote 10 symphonies. The numbers part escapes me.

There were four symphonies (alleged) which were never published or performed in Mahler's lifetime, and that is perhaps an allusion to one or more of them. However, even among the symphonies published and performed in his life time, there were more than three in a minor key.

Mahler wrote 8 symphonies, then Das Lied Von Der Erde, which is subtitled a symphony, then the 9th symphony, then a partial 10th symphony.

This sounds like a reference to Bruckner. In addition to his nine officially numbered symphonies, he wrote a symphony in F Minor known officially as the "Study Symphony" (without a number) as well as a symphony in D Minor known officially as "Symphony No. 0." If you add those to his Symphony No. 1 in C Minor, then you get a sum of 1.

A slight catch is that several of his other numbered symphonies are also in the minor key. Then again, the question doesn't say we have to add all of the minor key symphonies, just three of them.

A bigger catch is that Bruckner wrote more than ten symphonies, depending on how you count them. If you count only the officially numbered symphonies plus the Symphony No. 0, then you get ten; but excluding the Study Symphony messes up the "sum of 1" solution. If we do count the Study Symphony, then he has (at least) eleven. (I say "at least" because some historians argue that many of the other symphonies exist in several versions, some of them different enough from the "final" versions as to constitute a different piece entirely, sort of the same way Beethoven's various attempts at a Fidelio overture are now considered four different pieces.)