Translation:There are three patients in front of the hospital.
I think that "patient" doesn't quite do it justice; I believe that the noun usage of hasta is like the German der Kranke, i.e. "the sick one" or "the sick person".
So, a reasonable extension of adjective to noun with the meaning "the X one", "the one who is X", "someone who is X/something that is X".
"The sick patients are in the hospital" would be "hasta hastalar hastanede", I think -- "hastanede hasta hastalar" would be something like "in the hospital, the sick people are sick" or "in the hospital, they are sick people" while "hastanede hasta hastalar var" would be "There are sick sick people in the hospital".