Of the three, the swastika internationally is the most problematic symbol as far as international law goes. It is banned outright except for narrow historical exceptions in several Western countries (eg France and Germany). Other countries may not have banned the symbol per se, but they may have "hate speech" type laws, and a swastikas used for this purpose may also fall afoul of that.

The hammer and sickle is more comparable; it is indeed banned in a few (mostly post-Soviet) countries. But I've never heard of the hammer and sickle running into the same problems regarding hate speech laws, which was probably Reddit's focus here.

Though politics are involved, the LGBT flag represents identity and is not very comparable to major political movements with dodgy human rights records, or a symbol with a history of being behind "hate speech".