Is that 3-2-1 the numbers on the bag for concentration of Nitrogen, Phosphorus, and Potassium or is it some other ratio? Do you have a link to that product? If it is the N-P-K ratio it's higher than compost but not real high. You may need a fair amount of it. For example, the mushroom compost I saw at Lowe's was in the range of 0.5-0.5-0.2 for N-P-K. I can't remember if the P was 0.5 or 0.2.
I haven't grown sunflowers for several years, never did grow the mammoth. Goldfinches got the seeds before I did but that was OK, I was just growing for fun. I grew mine in with the corn, at the end of the rows.
Are you having issues or just asking in general? Unless your soil is really bad or something else is going on I'd expect you to be doing OK with a two week feeding of that. What stage are you in and how are they doing?