HELP with 3 days in MK

We are visiting WDW 3/24-4/1. MK on 3/26, 3/27 and 3/31. Girls are 7 and 10. Tomorrow is earliest I can arrange FP. Any suggestions on how to manage the attractions without tons of waiting? Should we try to stay in 2 lands per day? For example: Tomorrowland and Fantasyland on day 1, Frontierland and Adventureland on day 2, and Liberty/MainStreet on day 3???

We have done 3 days at MK for the last two trips and have done 1/2 park from RD to late afternoon on 2 days and then revisited favorites and shopped and had longer TS meals at resort or BOG or CRT on the 3rd day and stayed for Wishes and park closing. You will have plenty of time to do everything you want especially if you RD. You will have 9 MK FPP and is pretty easy to get 4th and 5th FPP same day for less popular rides IMO. We did Tomorrowland/Fantasyland on day 1, Frontierland/Adventureland/Liberty Square on day 2, and then rerides or whatever we missed on day 3. This time we decided to do only 2 days at MK, 2 days at AK, 3 partial days at E, and 1 day at HS.
ETA my kids were 13, 11, 5, and 4 last trip.

I think your best bet it to make up TPs (or use the pre-fab 2-day ones), then save day three for favourites or things you’ve missed.
When we do two days, our strategy (which doesn’t include any fast or thrill rides) is:
Day 1: Buzz, Winnie the Pooh, Fantasyland, then back through Tomorrowland for shorter-wait rides (people mover, CoP, MILF). FPP for a princess meet, ETwB, 7DMT, then 4th FPP usually Buzz on the way back out.
Day 2: Star with whatever we missed in Fantasyland, then continue counter clockwise for HM, and Adventure/Frontierland. Easier because my chickens won’t even go on Splash. We get FPP for HM, JC, and PP.
I’d make day 3 FPP for whatever you think they’ll want to do again, especially 7DMT if you can.

There’s a lot to be said for staying in certain lands for most of a day. It kind of heightens the flavor of those areas. But Fantasyland could probably use a day all its own, and Liberty Square and Main Street are probably too light on attractions to have a day just between them. Maybe:

Adventure and Tomorrow on one day (choose FP+ from among Space Mountain, Buzz Lightyear, Speedway, Jungle Cruise, Pirates of the Caribbean, and Magic Carpets). These two lands benefit the most from evening darkness, I think.

Liberty, Main Street, and Frontier on one day (choose FP+ from among Town Square Theater, Splash Mountain, Big Thunder, and Haunted Mansion)

Fantasyland on one day (tons of FP+ options)

A lot depends on how you want to tackle your days, too. Will you be there at rope drop? If so, at least one big attraction can be done first thing without FP+. Will these be full days? Afternoon break or no? Will you want to see the parade or the fireworks on one or more days? All these things will inform your FP+ strategy.

I can’t remember the last timeI spent 3 days in MK (1.5 is usually my max), but I agree with the concept of planning on seeing everything in 2 days and then using day 3 for re-rides and missed attractions.