First off, a refresher on critical volume:
Swim – 9/3 of the goal event distance per week
Bike – 8/3 of the goal event distance per week
Run – 7/3 of the goal event distance per week
All of these recommendations and those below apply to the final build weeks before your major “A” race. Everything leading up to that should be placed at a gradual slope starting at about 35% of the peak weeks 30 weeks out from the race. Some precautions: available volume during the peak weeks = the lower of:
1) The volume you can fit into your life logistically.
2) The volume you can handle at the beginning of the plan based on the fitness you currently have and the start volume that the above 35% gave.
3) 20% greater than your biggest weeks last year.
1) Take your available volume and allocate it to the swim bike and run using critical volume ratios. That is, if your race is IM, than when your bike mileage is at 200 miles, the run should be 40 miles, and the swim at 9,000 yards. This case is 2/3 critical volume for all three sports. This is also what I typically recommend as a minimum for any race distance; 2/3 critical. If your previous training does not put you in a place to hit this safely, you’re better off choosing a shorter race distance.
2) If critical volume is met in all three sports, add additional volume to the swim and bike up to 1.5 times critical in each. When bike hits 450, swim should hit 18,000.
3) Any other remaining volume should be added to the run up to 1.3 times critical there.
4) If there is still volume left, it should be added to all three sports equally using crtical volume ratios. However, at this point, you have gained the best advantage you could in terms of durability and any additional volume would be added in hopes of bettering speed potential. A more reasonable alternative is probably to increase intensity and leave volume as is.
Although, most folks have weaknesses in one of the sports, in my experience it still results in a better overall race result to work all three sports equally (assuming you have been doing triathlons for a while). Just work the weakness with a bit more focus. Chances are, that if you have a weakness, its just because you were not training it equally in relation to the others.