You control creatures that can tap to produce at least ({3} magic symbol) .
({1} magic symbol) available.
Steps
Activate Karn by paying ({1} magic symbol) , causing Isochron Scepter to become an artifact creature until end of turn.
Activate creatures you control by tapping those, adding ({3} magic symbol) .
Activate Isochron Scepter by paying ({2} magic symbol) and tapping it, casting a copy of To Arms! without paying its mana cost, untapping all creatures.