It all depends on what is meant by "Stage 3 compliant".
ICAO's General Assembly adopted a resolution in September 2001 which would allow the European Union to ban aircraft at noise-sensitive airports which it felt were just creeping under the Stage 3 limits -- by using hushkits for example.
Aircraft which only meet Stage 3 noise limits by a margin of 5 decibels can be banned at certain EU airports (those worst-affected by noise) because the EU feels that a "true" Stage 3-compliant aircraft is a lot quieter than a modified smoker. It's all about encouraging airlines to respect the spirit of the noise agreements, instead of trying to get off on a technicality.
Incidentally you'll notice that the MD80 isn't exactly flavour of the month in Europe, for precisely this reason. Although I believe that there's a company called Jet Engineering which is developing an MD80 hushkit which will put the aircraft into the ICAO Chapter 4 compliance zone.