Hello, I need help. My plants looked good from the beginning but now they are in the third day of 7 weeks of flowering and the heads are thin.

Considering where you are at in your flowering timeline, there is little you can do to materially improve your flower at this point (although you could certainly make it worse). I am not 100% sure what you mean by the heads being thin but I think you are referring to the denseness of the flower sites. The density of a flower site has multiple factors that come into play (genetics, humidity, and light). The top of your soil does appear to be rather dry, leading me to question the humidity in your enviornment. As a general rule of thumb: if you have good genetics and your humidity is appropriate, then giving your plants proper DLI will net you rock hard dense nugs every time.