In Uttar Pradesh, a three-year diploma holder can obtain a 12th equivalent certificate through several options:

1. **NIOS (National Institute of Open Schooling):** One of the most accessible ways is to enroll in the National Institute of Open Schooling (NIOS). NIOS offers a Senior Secondary Course, equivalent to the 12th grade, which allows students to choose subjects according to their interests and complete the course at their own pace. After passing the exams, you will receive a certificate equivalent to the 12th standard.

2. **State Board Exams:** Some state boards, including the Uttar Pradesh Board of Secondary Education, may allow diploma holders to appear for the 12th standard exams as private candidates. This option is useful if you wish to obtain a 12th standard certificate recognized by the state education board.

3. **Lateral Entry to 12th Standard:** In some cases, educational institutions may allow diploma holders to directly enroll in the 12th standard, especially if the diploma is in a related field. However, this option depends on the specific rules of the institution and the state education board.

4. **Distance Education:** Several universities and boards offer distance education programs where you can pursue and complete the 12th equivalent course.

Choosing the right path depends on your specific needs, such as whether you require the certificate for higher education or job opportunities.