If I have a temporary Karthus and Watchful Sentry on Dusk Rose Lab, would I draw 3 cards if I target my Watchful Sentry with the Battlefield?

To determine if you draw 3 cards, we must analyze the interaction between the Dusk Rose Lab, Watchful Sentry, and Karthus, Eternal during your Beginning Phase.

Short Answer

Yes, you will draw 3 cards.

Sequence of Events

1. Beginning Phase: At the start of your Beginning Phase, two events trigger: the Temporary keyword on Karthus (which would kill him) and the Dusk Rose Lab triggered ability. As the controller, you choose the order in which they resolve.
2. Choosing Order: You resolve the Dusk Rose Lab ability first.
3. Cost: As the cost to activate the Dusk Rose Lab ability, you kill your Watchful Sentry (which is at that battlefield).
4. Deathknell: Because Watchful Sentry died, its Deathknell ability triggers: "Draw 1."
5. Passive Interaction: Because you have Karthus, Eternal on the board, his passive ability—"Your [Deathknell] effects trigger an additional time"—applies to the Watchful Sentry's Deathknell.
   • The Deathknell trigger creates two instances of "Draw 1" on the chain.
6. Lab Effect: The Dusk Rose Lab effect ("draw 1") resolves as part of the lab's original ability.
7. Total: You draw 1 card from the Dusk Rose Lab ability, and you draw 2 cards from the doubled Deathknell effect of your Watchful Sentry, for a total of 3 cards.

Important Notes

• Karthus's Status: Even though Karthus is Temporary, he remains in play until his own Temporary trigger resolves. Since you prioritized the Dusk Rose Lab ability first, his passive is still active when the Watchful Sentry dies, allowing him to double the Deathknell effect.
• Cleanup: After the Deathknell effects resolve, the Temporary trigger for Karthus will resolve, and he will be killed (unless an effect prevents it), as he was not the unit killed for the lab cost.