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".
Yes, it should be clarified we are talking about the Nazi swastika.
Unfortunately in the West, the Nazi use of the swastika dominates the public perception of the symbol. (This is very unfortunate given the long historical tradition of the swastika in Asia and other cultures.)
As far as I'm aware, legally, context matters significantly in places where the symbol is banned; eg the Nazi swastika is outlawed in Germany because it represents a banned political ideology, whereas a Hindu swastika is not.
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".