When two substituents are already present on the ring, the third substituent's new location is relatively predictable.  If the existing substituents reinforce or the molecule is highly symmetric, there may be no ambiguity.  Otherwise:
# The most-activating substituent usually controls over the less-activating one.  
# In particular, ortho/para directors control over meta ones.  
# When multiple substituents are comparably activating, steric hindrance dominates regioselectivity.    
# In particular, the position between two substituents, each meta to the other, reacts last.