All positions are exact. Normaly with double or triple blocks there is almost no "time window". There is maybe +/- 1 second, but that is covered by lag most of the time. I suggest you try to zoom out your map so much, that you can see your garrisons and bandit camps at the same time. First send out 1st block general, and when he is on the way, select your second block general's garrison. Put the cursor on the camp he is suppose to attack, and than wait for 1st general to come on the right spot. Do the same for third or leader attack. This way you ensure you are allways on the right time frame for all attacks.
Hope this helps.