Employment Anniversary

I need some advice on how much notice I have to give my employer in order for me to leave my post. The language in my employment contract appears to give different answers from my friends (who are not lawyers) so thought I would put it here.

The relevant passage states:

"At any time, this Agreement may be terminated without cause by the [employee] xxxx with one month's written notice after a period of three months probation xxx [the employer] during the first year of employment of employee with the employer with notice of such increase by an additional month for each full year of employment. "

I just spent my third birthday - my question is - do I have to 3 months notice or am I required to give four months notice?

Thank you all.

3 months notice, as you still have to finish the fourth full year of employment