You have an amount of mana available equal to three minus the number of +1/+1 counters on Runaway Steam-Kin.
Steps
If Runaway Steam-Kin has three or more +1/+1 counters on it, activate it by removing three +1/+1 counters from it, adding ({R} magic symbol) ({R} magic symbol) ({R} magic symbol) .
Activate Cormela by paying ({1} magic symbol) and tapping it, adding ({U} magic symbol) ({B} magic symbol) ({R} magic symbol) that can only be spent to cast instant and/or sorcery spells.