Download codes have begun to go on Sale in Japan for Fire Emblem: Three Houses. On the back of the Download card, it reveals the download size for the game as well as amiibo support. It is not specified as to what the amiibo will do not has there been any new amiibo announced for the game. It is most likely that previously released Fire Emblem amiibo will have some function in game like unlockable outfits, Hero Units, or spawn chests with items or weapons.
A Nintendo Switch Online subscription will be required to use the online features. Not too much is known about this at the moment but Nintendo will most likely reveal what the online features consist of closer to launch. DLC is also mentioned on the Card which might suggest extra side-quests and missions may be on the cards. On top of that, in-game purchases are stated on the box. This may be linked to DLC though.
This information comes from a post on Reddit with an image of the back of a Download Card for Fire Emblem: Three Houses. It was later shared by Twitter user @theprinceofiris.
via Reddit: FE Three Houses Japanese download card
Provides the following new info:
1. DL version is over 11.9GB
2. Utilizes Nintendo Switch Online (most likely referring to Global Activity)
3. In-game purchases
4. Future missions, paralogues, and costumes will be released pic.twitter.com/LKrtd2eWAR
— Øscar @ 最終弾 発売中 (@theprinceofiris) June 24, 2019