Well Ok it was my presumption i could see S2, if i can't i will target S3. If S1 is visable and since its the more hurt of 1 and 3 ,i would target S1 in priority over the two,but if i can only see S3 i will Firebolt them.
I just live by the motto,Dead monsters tell no tales... and they don't get to attack back..So S2 i was trying to take out of the battle.