You can't activate abilities of cards in the target opponent's hand.

Abilities that create replacement effects, such as a permanent entering the battlefield tapped or with counters on it, are unaffected by Sen Triplets. Abilities that apply “as [this creature] enters the battlefield” are also replacement effects and are unaffected.

Activated abilities are written in the form “[Cost]: [Effect].” Some keyword abilities (such as cycling) are activated abilities and will have colons in their reminder texts.

Triggered abilities use the word “when,” “whenever,” or “at.” They're often written as “[Trigger condition], [effect].” Triggered abilities are unaffected by Sen Triplets.

Sen Triplets doesn't allow you to play more than one land during your turn or change when you may cast spells from the target opponent's hand.

The target opponent can't activate any abilities, including mana abilities.