Mill three cards, then return a land card or Elf card from your graveyard to your hand. If you can't, draw a card. (To mill a card, put the top card of your library into your graveyard.)
If there’s a land card or Elf card in your graveyard, you must return one. You can’t choose not to in order to draw a card.
Roots of Wisdom doesn’t target any cards in graveyards. You may return a land card or an Elf card that was just milled or one that was already there.