When Gruff Triplets dies, its second ability and Nim Deathmantle trigger.
Resolve the Gruff Triplets triggers, putting a number of +1/+1 counters on each other creature you control named Gruff Triplets equal to the sacrificed Gruff Triplets' power.
Resolve the Nim Deathmantle trigger, causing you to pay ({4} magic symbol) to return Gruff Triplets from your graveyard to the battlefield, and attach Nim Deathmantle to it.
When Gruff Triplets enters the battlefield, it triggers, creating two token copies of itself.
Activate Ashnod's Altar by sacrificing a Gruff Triplets token, adding ({C} magic symbol) ({C} magic symbol) .
Repeat.
Results
Infinite +1/+1 counters on each Gruff Triplets you control.