I was Genotype 1 so a bit different than Genotype 3. However, I was in the same boat in terms of being DET at week 4. Being DET at week 4 meant my SVR chances dropped to 64% even with 48 weeks (and that was the top percent as other trials listed a lower number). I did 48 weeks instead of 24. I attained SVR so it was definitely worth it to do a longer treatment in order to increase my chances.
You may be interested to read timothy's thread because it contains info you are asking. Here is a link to that thread.
http://www.medhelp.org/posts/Hepatitis-C/Is-it-really-necessary/show/1924112
Also, here are a couple more links:
"The 24-week course is especially recommended in the presence of steatosis (often associated with Genotype 3 infection), fibrosis stage two or more, high BMI and high viral load. In patients who do not achieve a rapid viral response (RVR) with combination therapy, an extended course up to 48 weeks should be considered. While not as definite as for genotype 1 patients, the presence of the CC variant of IL28b could help in the initial prognosis and the need for additional treatment, if an RVR is not achieved. "
http://hepatitiscnewdrugs.blogspot.com/2012/01/treatment-of-patients-with-genotype-3.html