Hi Cyndih70. Please try not to worry as I have seen bettas survive for a few weeks without food. Now, since your betta hasn't eaten in a while, it has essentially been fasting itself. Which is good because if it was constipated, this would have given ample time for the food to absorb moisture and pass through your betta by now.
The lethargic activity and unwillingness to accept food can be a sign of stress or illness. The first thing to check would be your water conditions. With the use of a test kit you should test for the presence of ammonia, nitrite and nitrate. Ammonia and nitrite levels should be at 0 parts per million (ppm). Nitrate levels should be below 40ppm although more preferably below 20ppm. If you dont have a test kit, I would suggest getting the API freshwater test kit. It retails for around $25 - $30 in stores but it is a good investment and lasts a while. Try searching nitrogen cycle for more information.
In addition, you said you use API Stress Coat. This is a good water conditioner but it wont work properly unless it is properly dosed. I don't personally use it but I have read online that the manufacture recommended dosing quantity is 2 tsp for every 10 gallons. This would equate to 3ml of Stress Coat for your 3 gallon tank. This is important because any presence of chlorine can be causing harm to your betta. If you're doing a partial water change (70% as you said) you would have to calculate for that as well. 70% of 3 gallons is roughly 2 gallons, and this would equate to 2ml of Stresscoat for the new water you will be adding during water changes.
Now on to feeding your betta. I would suggest trying frozen foods for your betta. In particular, frozen daphnia. Daphnia is an aquatic crustacean found in freshwater habitats. The reason I suggest daphnia is because it is natural to the bettas habitat and contains good nutrition and fiber that your betta needs. The fiber in the exoskeleton of daphnia will help to purge your bettas GI tract (basically from the stomach to the point of expulsion from the body). So in other words, the high moisture content and fibrous material act like a natural laxative for your betta, which is very useful if he is still experiencing constipation, while still providing the necessary nutrients for proper bodily function.
You can find daphnia in the freezer section of your LFS, petco, or petsmart. (Petsmart is where I find Hikari brand). It comes in frozen packs which you can cut out one section at a time to thaw and feed to your betta. I like Hikari brand as it is sterilized and frozen in pure water. Sterilization is extremely important especially for a potentially sick fish because we don't want to introduce any harmful microbial organisms to a sick fish. Feeding should consist of small portions. You can use a toothpick to stir around in the daphnia and then dip the toothpick into the water once or twice. The daphnia are small and easily eaten by bettas and they will entice the natural instinct for eating since they are a natural food. You may also want to turn your filter off just when feeding because the daphnia are so small that they will be swept away by the current.
If your betta does not improve with these suggestions over the next 3 or 4 days. You may want to consider medicating, which is always a last resort! If your betta is sick and has no obvious signs of bloating, cut/soars, or infection, you should consider using a wide-spectrum antibiotic. As previously suggested, keep a close eye out for fecal matter (poop) and take note of what it looks like as it can tell you a lot of what may be happening inside your betta. If you choose to try my suggestions, we can go over medicating when and if it comes to that point.