({3} magic symbol) ({R} magic symbol) , plus an additional ({X} magic symbol) available, where X is equal to three minus the number of +1/+1 counters on Runaway Steam-Kin available.
Steps
Activate Spellbinding Soprano's encore ability by paying ({3} magic symbol) ({R} magic symbol) and exiling it from your graveyard, creating a token copy of it with haste for each opponent you have.
Declare each Spellbinding Soprano token as an attacker.
Each Spellbinding Soprano token triggers, causing instant and sorcery spells you cast this turn to cost ({1} magic symbol) less to cast for each Spellbinding Soprano token.
If Runaway Steam-Kin has three or more +1/+1 counters on it, activate its ability by removing three +1/+1 counters from it, adding ({R} magic symbol) ({R} magic symbol) ({R} magic symbol) .
Cast Sprout Swarm by paying ({1} magic symbol) and tapping a green creature you control.
Runaway Steam-Kin triggers, putting a +1/+1 counter on itself.
Resolve Sprout Swarm, creating a 1/1 Saproling creature token and returning Sprout Swarm to your hand.