Welcome to the forum and thanks for your question.
However, like many people, you have come to the assumption that hepatitis C virus (HCV) is an STD in the same manner as other traditional STDs or hepatitis B. It is not. Heterosexual transmission is rare, if it occurs at all, even with unprotected vaginal sex; and oral sex is not a known transmission risk. The only proved scenarios for sexual transmission of HCV is among gay men who participate in traumatic rectal sexual practices -- and even here, it appears to be an important problem only in HIV infected men. Here is the link to a thread with a bit more information about it; you can find others through the search link.
http://www.medhelp.org/posts/HIV-Prevention/Testing-/show/1668453
Given that your partner has a known HCV infection, I can understand your desire for testing for reassurance purposes. You definitely can expect negative results. However, 3 weeks seems a bit early. You might check with your doctor or clinic about recommended intervals; 6 weeks might be more reliable. (Because it's not an STD, we don't try to keep up with detailed recommendations about HCV testing.)
Best wishes-- HHH, MD