Well it would depend on the contract type and if you are invoicing the company during the 3 months. If the company are going to pay you a sum each month without deductions then you will need to contact the inland revenue and ask to pay your contributions.
But if the company are paying you and are deducting tax and NI from the salary then you will not go self emp
If this is your only contract, you will probably fall under the IR35 regulations and be classed as employed, no matter whta you thnk you are.
Check your status with your local tax office (they wont want to know who you are). Why would you want to go self employed unless you really are going to do it properly - Do you really want automatic self assessment for the next gazillion years, even when you get a full time job?
Actually IR35 is such a deep and confusing piece of legislation that you may or may not fall inside IR35 depending your contract, office hours, who and how you are managed, duration of contract hours, whether you work on or off site, if other people will be used to work on the contract and the list goes on and on and on.
Speak to your accountant about your situation as you may beable to declare the monies at the end of the year.
This is true but the IR35 liability for NI and tax (if HMRC decides the person is indeed an employee) lies with the employer, not with sally, who by this time has long since had her money and disappeared.
None of the answers above shows any understanding of IR35, I;m afraid - not even Buildersmate, who is usually very expert. 99.9% of the time, IR35 applies only to people who are working through Ltd companies of their own. Assuming you're not doing this, then practically speaking it's the business you're working for who will decide whether you're their employee or not. HMRC can tell them they're wrong but they'll very rarely over-rule PAYE treatment. At the root of your question is whether a person can decide for themselves what their employment status is - they can't.