Not sure what you read here.
Occasionally for newer devices that follow on from older devices or revisions of older devices you find so called unpopulated headers. Sometimes these are things from legacy devices that are not present on that revision (most notable around here is some of the gamecube stuff is still available on the gamecubeless wiis), and other times they can be interesting for hackers if they are debug ports or something like that. The DS however lacks anything like this to use a GBA link port and while I don't know for certain I would be absolutely stunned if the 3ds had such functionality.
There is no easy way to emulate link functionality on a 3ds without writing a GBA emulator for it (given injection methods are popular and not just a silly thing some people did says about how much you need to know about the effort people care to put into writing emulators on the 3ds), and no simple "buy these 10 chips, solder it together accordingly" like you might have seen with something like vbalink-real.
Depending upon what you need it for then it is probably easier to dump the saves, use vbalink (or vba-m if it has it, or any other link capable emulator) and chuck the saves back over.
