Yes it is. But you'll have to patch the NDS ROMs to read your GBA pokemon SRAM savegames off the 3in1 because the original cartridges use EEPROM. Search the forum I think there's a thread on this.
I assume you're referring to the linkage between slot1 and slot2 pokemon games, if you're referring to linking GBA pokemon games with the DS in 2 separate consoles via wireless and trade, it isn't possible because the NDS, despite having wireless, can't do GBA wireless function.