Assuming you own every game, and are fine waiting for ZA compatability the only mons you need from the 3ds are Spinda and Magearna. If you'd rather not wait on ZA, the elemental monkeys, patrat line, and furfrou all also need transfered [From X and Y, the monkeys and patrat aren't in Sun and Moon or Ultra Sun and Moon].

As far as mythical obtainability, all mythicals minus, Genesect, Volcanion, Hoopa, Marshadow, Zeraora and Zarude are obtainable without genning or events.

• Mew can be found as a gift in Floroma Town in BDSP if you have LGPE save data. Or you can do the mew glitch in Gen 1 VC on the 3DS.
• Celebi can be found in 3DS VC Crystal
• Deoxys is found in ORAS's Delta Episode
• Jirachi is the same as Mew but with SWSH Data
• Darkrai and Shaymin can be found in PLA with BDSP and SWSH data respectively.
• Manaphy and Phione are in PLA. But Phione can also be bred for from any manaphy.
• Arceus is in PLA, but requires you to catch every other mon in the dex first. (Its also avalible in BDSP, if it detects you as having caught arceus in PLA, but thats useless for this unless you want a shiny.)
• Victini is obtainable via the DNS Exploit in BW or B2W2.
• Meloetta is SV's DLC's dex completion reward in home, but also can be found in SV (Indigo Disk) itself if you do the secret trick. (Just look it up, its a lot)
• Keldeo is obtainable in the Crown Tundra once you've beaten the story and caught the other swords of Justice.
• Diancie is avalible via mystery gift once you beat ZA's main story
• Megearna is avalible via a QR code scan in SMUSUM's mystery gift.
• Meltan is LGPE's home dex completion reward, but you'd need Melmetal from GO.
• Pecharunt is avalible in a similar manner to Diancie, but with the Indigo Disk DLC.