I've heard you can hack it on your 3ds, not sure though!

not offically but there my be fan games and streaming it from ur wii u

Unfortunately no, however there MIGHT be some fan games out there on the 3DS that are similar to splatoon but even if there were any, it would most likely be impossible to play online unless if it had pretendo support, however I'm pretty sure pretendo don't support any fan games

NO

There is a port of Splatoon in development for the 3DS under the name, "Splatoon Legends." They have a Youtube channel & a Discord server.

Yes! it is not fully out yet but the guy above me explains it well