What do you mean 3 months in advance?
It's obviously better to have to done before you start your application. I say the sooner the better. For example, I found out everything about Optometry admissions on my own through this forum, had I know about OAT, I would've taken it a year before my applications. That way, if I wanted to get higher scores before I apply to schools, I would've had the chance. Since, I'm applying for Fall 2013, my only chance for OAT is this summer before September 1st 2012.
The sooner you start studying for it and doing it, the more advantageous it is for you.
Hope this helps you and good luck!