Faffymum, I am so sorry you are feeling so depleted and tired. Three weeks is when babies normally "wake up" a bit and they tend to get more needy. You are doing GREAT! Just keep on doing what you are doing, feed on demand, sleep when baby sleeps (yes, really, leave EVERYTHING else - you need sleep). Your body will soon catch on to the up in demand.
I would also strongly suggest you take your baby to a chiropractor. I believe each and every baby can benefit from chiropractic adjustment. How was he born?
Formula is a downhill path, I'm afraid. The moment you give formula, he will drink less on you, supply will drop, you will give more formula and before you know it, your breastfeeding days are over.
Just hang in there. It will soon get better. Drink another eglonyl if you think it will help. Demand feed, demand feed, demand feed. Eat proteien rich foods and drink lots of liquids. Wear him in a wrap if you can so he can settle (You can literally take a 6 meter stretchy material and make your own stretchy wrap. You don't even have to stitch it. Otherwise you can buy one and it can reach you in two days. I can recommend if you like.)