Now the third week of flowering has begun. What strength should the solution be optimal? 

i would go 2 the feed chart of what ever nutes u r using and u should find the PPM range for the week u r n .... my 2 cent 

For the 3rd week of flower I use an EC of about 1.3-1.5 depending on how tolerant the plant is. So that would be like 650-750 ppm.