When Threefold Thunderhulk dies, Nim Deathmantle triggers, returning Threefold Thunderhulk from your graveyard to the battlefield, and attaching Nim Deathmantle to it.
When Threefold Thunderhulk enters the battlefield, it triggers, creating a number of 1/1 Gnome artifact creature tokens equal to its power.
Activate Phyrexian Altar three times by sacrificing three Gnomes, adding ({3} magic symbol) .