I'd recommend your third option (3MK/1HS/1Epcot), split up sort of like this (based on my children's preferences on their first trip, at ages 4 and 6):
MK - 3 days, bookending your visits to the other parks. 1 day Adventureland, Frontierland, and Liberty Square (except for Tom Sawyer Island). 1 day Fantasyland, Tomorrowland, and either: (1) Tom Sawyer Island/ or (2) 2-3 character M&Gs. Then on the third day (a half-day, really), spend it re-visiting all of their favorite attractions. For the other half of that third day, visit Disney Springs, go to a special dinner like the Hoop-de-Doo Musical Revue, or have an extended swimming/relaxation break at your resort.
Epcot - 1 day. Future World in the morning, skip to World Showcase after 11am (and don't linger there too long, because the kids will probably be quite bored there except at the 2 pavilions with "rides," no matter how smart they are or how interested they may be in learning about other places and cultures), and back to Future World in evening to finish up what you couldn't squeeze in before. (Alternatively, save World Showcase for the evening, and enjoy dinner at one of the many excellent restaurants there.)
HS - 1 day, divided however you see fit.
EVERY DAY (obviously YMMV, but this is what works for us) - Arrive at least 30 minutes before rope drop. Tour until lunchtime (about 4 hours), at a fairly steady pace, stopping for breaks as needed (especially if the 2 1/2 year old is potty-training). Have lunch and head back to your resort for an extended nap/swim break. (The nap break is for everybody -- even my DH and I take naps at WDW, although we never do at any other place or time!) Don't go back to the parks until around 4 or 5pm, as crowds and heat dissipate, and tour for another 4 hours at a more relaxed and leisurely pace, stopping for a TS dinner, if desired. When the kids start to droop around 8 or 9pm (unless it's the last night of your vacation, when anything goes), head back to the hotel, so they'll be rested enough to get up for another rope drop the following day. Getting there at rope drop and having a total of 8+ hours to tour each day (making good use of post rope-drop time and FP+) ensures that you'll get to do practically everything in every park, with a minimum of wait time, and with the kids at their maximum rest-and-refreshment level at all times. I'm sure this method isn't fool-proof, but I can testify that it's allowed us to visit WDW as a family for a week with no meltdowns whatsoever!
EDIT: I just realized I inadvertently substituted Epcot for AK in making up that plan. Candidly, I'll echo
@MansionButler84 's suggestion, and would suggest converting the HS day to an AK day, and keeping Epcot in the plans. Unless there's something the kids absolutely HAVE to see at HS, Epcot actually offers more attractions for their age group than HS does, even if you ignore World Showcase. (HS may have more shows, but AK also offers some great shows -- I would argue, even better than the ones at HS -- and there's only so many shows a toddler can take!). It's a shame to miss Toy Story Midway Mania or Star Tours at HS, but the fact is that the other parks offer comparable shooter/simulator experiences (e.g., Buzz Lightyear's Space Ranger Spin, Mission:Space).
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