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.
In 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 probation period is determined on the basis of the nature of the job and the extent of the complexity of the organizational set up.
How quickly a probationer acquires the practical skills required for the job and orient himself with the organizational work culture depends upon his personal abilities and willingness. But, he should prove his performance and behaviour to the satisfaction of the employer within the period of probation failing which either his probation can be extended or his services can be simply terminated as mentioned in the contract of 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 thus confirmed, he 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.