The narrative in the opening part of the post gives an impression as if the poster feels that the confirmation of the poster before the expiry of the period of probation is a breach of the contract of employment, which, in consequence, gives him a right to stick to the notice clause applicable to a probationer since his resignation also takes place within the probation period.
No, it is not so.
Understanding the Period of Probation
In the employment realm, the objective behind the period of probation of a newly recruited employee is to assess his suitability for the job and to ensure his orientation with the general work culture of the organization within a span of a period so fixed in the contract of employment or service regulations of the organization. Normally, the length of the probation period is determined based on the nature of the job and the complexity of the organizational setup.
Factors Affecting Probation Confirmation
How quickly a probationer acquires the practical skills required for the job and orients himself with the organizational work culture depends upon his personal abilities and willingness. However, he should prove his performance and behavior to the satisfaction of the employer within the probation period. Failing this, either his probation can be extended, or his services can be simply terminated as mentioned in the contract of employment.
Employer's Prerogative in Confirming Employment
It would automatically lead to the implication that if satisfied, the employer can also confirm a probationer before the expiry of the period, which is a prerogative of the employer. Once confirmed, the probationer is bound by all the rules of employment, including notice on unilateral termination of employment.
Therefore, in the present case, the poster has to buy out the notice period of 3 months applicable to a confirmed employee of the organization.