Such awesome advice here already!

I agree with Tidbits and MelInKansas. Those pairing make a lot of sense. You will be amazed at how freeing it is to let your oldest, Abby, soar! She is going to love RevtoRev, or MTMM. I'd take the timeline of history out of it, and place her according to where she places best according to the first page of the placement chart.

For your next pairing, Joshua/James, PHFHG with extensions would be a good fit. For your last pairing, finishing out LHFHG will be a good fit. I know it seems hard to believe, but I think that doing these 3 guides at once is going to be easier than trying to combine dc who can't combine well. Your LHFHG duo will be done in 1 1/2 hours or less, your PHFHG will be done in 3 hours and your time will be quite minimal, your RevtoRev or MTMM dd will be done in 4 to 4 1/2 hours, and your meeting time with her will be around an hour or a bit more (unless you add a bunch to her HOD day, or substitute other math/LA, etc. that takes longer). This is all VERY manageable. I think you would find this to be an incredible breath of fresh air! I am finishing out our year with HOD's High School World Geography, Resurrection to Reformation half-speed (did CTC full-speed prior and finished early), and Beyond Little Hearts. We LOVE our days! They are very doable - even for this mama that works part-time and has a dh that travels - even with a lot of health concerns this year with my family. God is good! I think you would find these placements to be solid, but check out the placement chart to be sure...
http://www.heartofdakota.com/placing-your-child.php
In Christ,
Julie