Question regarding Nether Shadow

[PhilipJFry]

Nether Shadow reads:
"At the beginning of your upkeep, if Nether Shadow is in your graveyard with three creature cards above it, you may put Nether Shadow into play."

Do the three creature cards need to be above Nether Shadow when the ability triggers, or when it resolves, or both? 

The answer is relevant for decks using multiple yard affecting abilities, where this event (3 creature cards above it) may be true at some points during the upkeep but not during others.

Thank you for your responses.

[Roxas]

Nether Shadow has triggered ability that triggers at the beginning of your upkeep only if it is in your graveyard with three creature cards above it. It will not trigger if both of those conditions are not fulfilled at the beginning of your upkeep.

[Clariax]

Do the three creature cards need to be above Nether Shadow when the ability triggers, or when it resolves, or both? 

Both.  The triggered ability on Nether Shadow has what's called an intervening 'if' clause (glossary entry below).  When the trigger condition occurs, beginning of upkeep, if the 'if' condition is not true, the ability doesn't trigger at all.  If when the triggered ability resolves, the 'if' condition is not true, then the ability does nothing.

Intervening "If" Clause: Triggered abilities with a condition directly following the trigger event (for example, "When/Whenever/At [trigger], if [condition], [effect]") check for the condition to be true as part of the trigger event; if it isn't, the ability doesn't trigger. The ability checks the condition again on resolution. If it's not satisfied, the ability does nothing. Note that this mirrors the check for legal targets. Note that this rule doesn't apply to any triggered ability with an "if" condition elsewhere within its text. See rule 404.3.