({1} magic symbol) plus either ({B} magic symbol) , ({R} magic symbol) or ({G} magic symbol) available.
Steps
Activate Yahenni twice by sacrificing Gruff Triplets and Slimefoot and Squee.
When Slimefoot and Squee and Gruff Triplets die, Pitiless Plunderer triggers twice and Gruff Triplets' second ability triggers once.
Resolve the Pitiless Plunderer triggers, creating two Treasure artifact tokens.
Resolve the Gruff Triplets trigger, putting a number of +1/+1 counters equal to Gruff Triplets' power on each creature you control named Gruff Triplets.
Resolve the Yahenni abilities, giving it indestructible until end of turn.
Activate two Treasures by tapping and sacrificing them, adding two in any combination of ({B} magic symbol) , ({R} magic symbol) or ({G} magic symbol) .
Activate Slimefoot and Squee by paying ({1} magic symbol) ({B} magic symbol) ({R} magic symbol) ({G} magic symbol) and sacrificing a Saproling.
When the creature dies, Pitiless Plunderer triggers, creating a Treasure artifact token.
Resolve the Slimefoot and Squee ability, returning it and Gruff Triplets from your graveyard to the battlefield.
Slimefoot and Squue and Gruff Triplets' second ability triggers.
Resolve the Slimefoot and Squee trigger, creating a 1/1 Saproling creature token.
Resolve the Gruff Triplets trigger, creating two token copes of it.
Activate Yahenni by sacrificing a Gruff Triplets token.
When the Gruff Triplets token dies, Pitiless Plunderer triggers, creating a Treasure artifact token.
Resolve the Yahenni ability, giving it indestructible until end of turn.
Activate two Treasures by tapping and sacrificing them, adding ({1} magic symbol) ({B} magic symbol) .
Repeat.
Results
Infinite +1/+1 counters on each Gruff Triplets you control.