If someone failed in one subject in bcom part 1so is he eligibel to go in bcom part 3?

No, a student who fails in one subject in B.Com Part 1 is generally not eligible to proceed to B.Com Part 3. Most universities require students to pass all subjects in a given year before advancing to the next level. However, policies can vary by institution, so it's best to check with the specific university for their regulations regarding promotion and re-examinations.