Whether you took a lower salary than you would have liked to get a job with the company, or you didn't realize until you started working with the company that your skills and qualifications are worth much more than you're being paid, it's not unheard of for a probationary employee to request a salary raise after completing the first few months of employment. Using your past performance to predict your future performance, along with solid proof of your achievements to date, you may be convincing enough to lobby successfully for a salary increase after the probationary period.
Mark your calendar for a date approximately two weeks prior to the end of your probationary period. Gather your job description, work accomplishments, supervisor and manager comments, and your resume. Review your job performance throughout your probationary period and prepare a narrative about your work and what you believe lies ahead in terms of future performance and tenure with the company.
Prepare a convincing argument for a salary raise based on your performance to date. Give concrete examples of your accomplishments, and if possible, produce evidence of your work, such as writing samples and feedback from supervisors, or evidence that you've been successful in completing your assigned duties and responsibilities.
Research labor market information concerning your occupation. Since the time you joined the company, there may not have been much movement in the labor market concerning the number of workers with skill sets similar to your own. However, you may be able to use labor market projections to support your request for a higher salary. For example, if your research indicates the presently saturated labor market will eventually dry up and leave very few people with your qualifications, include that in your request for a higher wage after your probationary period.
Rehearse your argument -- although not in an adversarial tone -- until you feel comfortable explaining in an articulate manner why you believe you deserve a salary increase after just 90 days or a short introductory period. Express your enthusiasm and that your full engagement and motivation underlie your desire to achieve even higher performance as your tenure with the company increases. Remind your supervisor and the human resources manager that you have demonstrated promise and that past practice is the best indicator of future success.