Just like usual. The top trigger resolves first, and your opponent can at this time choose to pay 3 life or not. If he doesn't, the creature card is returned to your hand. The next trigger offers him the same choice, and if he paid before and thus the creature is still in the graveyard, he can pay another 3 life to keep it there. If doesn't pay for any of the triggers, the creature is no longer in the graveyard and paying life for subsequent triggers is rather pointless, but still possible.
Helm of the Host
Just like usual. The top trigger resolves first, and your opponent can at this time choose to pay 3 life or not. If he doesn't, the creature card is returned to your hand. The next trigger offers him the same choice, and if he paid before and thus the creature is still in the graveyard, he can pay another 3 life to keep it there. If doesn't pay for any of the triggers, the creature is no longer in the graveyard and paying life for subsequent triggers is rather pointless, but still possible.
Former Rules Advisor
"Everything's better with pirates." - Lodge
(The Gamers: Dorkness Rising)
"Any sufficiently analyzed magic is indistinguishable from science."
(Girl Genius - Fairy Tale Theater Break - Cinderella, end of volume 8)