Hi Coquito,
I lived on a simple, mostly Boost Plus and egg-beater omelets, diet for several weeks after treatment ended, and things did not taste very good for a while (I ate everything using plastic utensils due to the awful metallic taste when I used metal utensils). I had no appetite for 3 months or so and only ate because I knew I had to. My appetite is much better now at 5 months and my cuisine has expanded to just about where it was pre-treatment. And I only recently started getting enough real food to stop drinking a Boost or two a day. Initially, sweets like chocolate and candy bars were totally repulsive to me - they are not so bad now although I still don't care for them. However I do like cake, yogurt, and fruit - a little sweet but not too sweet.
I am only now starting to gain some real weight. I'd say I've put on several pounds recently (although I didn't lose that much during treatment).
I can only recommend patience - three weeks is way to early to see much progress. The important thing is that you are eating, and you need to make sure you find foods that you can tolerate ("tolerate" being the operative word for a while) AND that have a lot of calories.
Best wishes,
Chris