import re
 
pattern = r"^.*?(?<!\d)(\d{3})(?!\d)"
 
s = ("\"Depart: NYC (etd 9/30), NJ (etd 10/4)\n"
	"Arrive LAX\n"
	"Rate: USD500, 700P\"\n")
 
m = re.search(pattern, s, re.DOTALL)
if m:
	print (m.group(1))