If the individual concerned was indeed sentenced to three years imprisonment, then he is prohibited for life. In which case the police cannot grant a certificate.
There is a right of appeal to have the prohibition removed and the police would be invited to be represented. In both such cases I experienced, the police were asked whether there was any objection. In both cases, there was no objection and the Court lifted the prohibition, one of which was five years and the other life.
There is rightly room for rehabilitation of those who have genuinely changed their ways.
If the individual was sentenced to three months but less than three years, he would have been prohibited for five years from the date of his release.
If the current application was made more than five years from the date of his release and refused by the police, then the normal route of appeal is available against the decision.
As others have suggested, he needs to get competent legal advice.