EXPLAIN
SELECT 
	count(*)
FROM 
	Properties LEFT JOIN 
	(Apartments INNER JOIN ApartmentModels 
             ON(Apartments.ModelID = ApartmentModels.ModelID AND Apartments.PropID =  1 ))
	ON (Properties.PropID = Apartments.PropID)
WHERE 
	Properties.PropID= 1 

table             type   possible_keys   key    key_len ref                rows     Extra 
Properties        const  PRIMARY       PRIMARY    4    const                 1      
Apartments        ALL      NULL        NULL     NULL  NULL                13915    
ApartmentModels  eq_ref  PRIMARY      PRIMARY     3   Appartments.ModelID   1   Using index