My Pro Coach training plans range from 8 to 40 weeks in duration, so the best thing to do is count back from your "A" target race date and then choose a plan to fill that gap. You can fit your other races into this timeframe.
The training plans all take you right up to race day (aside from the Winter Base Training plans). The longer the plan the easier it starts and the more gradually it progresses. So that has a bearing on the difficulty level you choose. For example a 12 week plan starts off at a higher level than a 36 week plan because with 12 weeks left there's not as much opportunity to build up slowly. So if you're choosing a short plan, err on the side of caution when picking the difficulty level.
Your choice of Novice, Intermediate or Advanced level depends on 3 things: Your current fitness, your time availability and your goals. Most importantly, your current fitness and your time availability.
The Novice plans take less time each week, followed by the Intermediate plans and then the Advanced plans which take most time. All the plans are designed for amateur athletes, and can be fitted around a Monday to Friday 9-5 job.
If you have a busy job and a young family, I would always opt for Novice or Intermediate level plan regardless of your level. Otherwise you'll struggle to fit everything in (unless you have lots of flexibility).
Only choose an Advanced plan you're already a triathlete with at least 18 months experience. Choosing an Advanced plan won't necessarily turn you into an advanced athlete if the plan is too hard for you from the start. In that case you'd be better off with an Intermediate plan.
You can preview each training plan and in the description there are screenshots of the first and hardest fortnights. Have a careful look at these and ask yourself if you're capable of a) fitting them in and b) fit enough to do the workouts.
You'll improve quickest by having the right training plan for your availability and current fitness.