The fact that the dryer takes so long to dry indicates either an air flow problem or a "heating" problem. You don't say weather the units gets hot or not but you do say that it has been giving you this problem for 3 years so I'll assume that it does heat. You should make sure that there is air flowing through the dryer by just going to where the dryer is exhausted to the outside after turning on the unit and see if it is blowing hot air out. If there is good airflow but it is not hot you may have a bad element or thermostate and you should contact a professional to diagnose the exact problem and repair it. If there is not a strong warm airflow from the exhaust then you probably have an airflow restriction of some type. The first thing to check is the lint filter. Every dryer has a lint filter that should be cleaned out after each load. Check you owners manual to find the location of yours and make sure it's clean. If it is plugged, clean it and check the airfow again. If that does't solve the problem, you need to determine if the dryer vent hose is kinked or plugged. The vent hose is usually located at the bottom of the back of the dryer and are usually only attached with a clamp of some type. Just disconnect the dryer vent hose from the back of the dryer, turn the dryer on and check the air flow at the dryer exhaust. The there is little or no air blowing out of the dryer it could be a defective fan. This is a complicated repair and should be done by a professional. If the exhaust coming out of the dryer is strong you know the problem is with the exhaust hose so check for kinks, crushed duct or blockage and repair it then re-connect the exhaust duct to the dryer, turn on the dryer and go back out to the outside exhaust and see if it is now blowing strong. GOOD LUCK. I hope this resolves your problem.

Try running dryer with venting disconnected to eliminate that as the problem. Thanks, please leave feedback or contact me again for more info if you need it.