The only cards allowed in 3 card blind events are vintage legal cards, plus Shahrazad. Restricted cards are considered legal for this purpose, and more than 1 copy of a restricted card may be played in a submission.
If you are not sure whether a card is legal, go to Scryfall and search for the card. If the Scryfall page says that its legal in vintage, then it is legal.
Dungeons are legally considered cards, and therefore will not be put into the command zone when venturing into the dungeon or the Undercity.
New sets are added to the pool if the prerelease event for that set happened within or before the submission window for that round. If it a supplemental set with no prerelease date, check Scryfall. All new cards within a set must have also either been previewed before the submission window opened or have been on Scryfall before the submission period opened.
The following vintage legal cards are banned in the format. For a more extensive breakdown of the banlist, see the annotated banlist tab in the sheet below.