I have long, irregular periods too. I'm using the FAM method. (We are going to start trying in a few months, right now I am trying to determine if I am ovulating). I would really recommend that method, since it explains when you are ovulating and is very inexpensive. I am just using the temperature taking, because the cervical fluid method sounds a little too confusing for me right now.
I don't know of anyone on any method who could say whether or not you could get pregnant within three months. If you are ovulating less than other women (like I maybe), it will likely take you longer than average. Average time is 1 year. I wouldn't try to get treatment after only 3 months, remember, for you that is only one ovulation. The treatment can cause other problems. Honestly, start figuring out when you are ovulating, and then try for those times. Give it more than three months.