While it is generally admirable (and well received) to start an entrepreneurial journey I don't think it is the right time (or business)
1) It is very difficult to make a true impact in three months
2) Starting a new business prior to applying for an MBA will lead to the question - why? If the business does well how will you manage to keep it goingwhile you are studying? Who will take care of your Clients? (The MBA is VERY time consuming and schools do now want you working on a pre-existing business at the same time)
3) There are a lot of business consulting practices and it takes a long time to build a good Client base. It is a multi year journey and starting it now might show a lack of forethought to the AdCom.
4) The admissions process itself is time consuming - I don't recommend starting a new company, studying for the GMAT, and preparing the application all at the same time
That said - every situation is different. Is there a specific reason you need to start it now? (e.g. a Client lead you may lose later?) Is it that you want to show an entrepreneurial side? If so you could start a small fun side business while keeping your day job.
In short - I don't advise it, but as with everything context matters.
Pamela Jaffe
MBA Admissions Consultant
The Jaffe Advantage