The idea of having such a popular game such as Splatoon 2 ported for the Nintendo 3DS is definitely something that many people would love to see.  However, the chances of it actually happening is something that is questionable.  There was never a 3DS version of the original Splatoon game since it was exclusively for the Wii U.  Splatoon 2 is considered to be a Nintendo Switch exclusive and making it available for any other game console could possibly diminish the value that the game already has on the Nintendo Switch.  

If Nintendo really wanted Splatoon 2 to be available for the Nintendo 3DS they would have ported the game for the handheld 3DS device back in July 2017.  If Splatoon 2 was ported for the Nintendo 3DS on one hand it would help the game become more popular in terms of sales.  On the other hand having Splatoon 2 on the 3DS could possibly turn some attention away from the Nintendo Switch which is very hot in the gaming market right now. Making Splatoon 2 a Nintendo Switch exclusive was probably a good decision on the part of Nintendo in the long run.