Spawning Pit has at least two charge counters on it.
({1} magic symbol) available.
Steps
Activate Spawning Pit's second ability by paying ({1} magic symbol) and removing two charge counters from it, creating three 2/2 Spawn artifact creature tokens.
Activate Phyrexian Altar by sacrificing a Spawn, adding ({1} magic symbol) .
Activate Spawning Pit's first ability twice by sacrificing two Spawns, putting two charge counters on Spawning Pit.