I am considering doing all 6 mod in 3 years to give me time to repeat the cycle with my oldest, but I am not locked into this plan at this time. I will be doing mods 1 & 2 this year and will see how it goes from there. I am going to do Bible daily following the SCM Mod lesson plans, For History and Geography we will be primarily following my own plans, although I will be using some SCM recommended resources (The Golden Goblet, Boy of the Pyramids for 3rd grader, Pharaoh's of Ancient Egypt, etc.) I am treating Bible as a seperate subject and will not be concerned to carry over lessons from Mod 2 to next year when we start Mod 3 History. I know that the amount of reading is going to signifiantly increase as we progress through the mods, but I am confident that this will still be quite doable. We may drop a book here or there, or choose to focus on certain events more than others. We are Canadian, so we will be focusing on Canadian History in Mod 5 & 6 which means I will not be using most of the American History books recommended in those mods.
IF I were to be doing a 3 year cycle as late in the game as you are, I believe I would separate the Bible and scheduel that over 3 years as a separate subject, combine the Ancients (1-3) into one year, then devote 24 weeks or 2 terms each to the remaining mods(4-6).
Another option -- especially if you must have American History and you are most concerned about covering it adequately -- is to start with Mods 5 & 6 with the plan of covering them in 24 weeks (2 terms) each. This way if you need to spend more time, you can still carry over into the following term. Then, in the remaining terms of the second year and the final year you can cover the remaining mods according to interest and time available without the pressure of "We Must Complete to Graduate". Even if you don't cover everything in Mod 1-4, your child will still have covered more history in a more memorable and enjoyable way than the majority of PS students ever will.