I may be an outlier here, but I would say the first thing to plan is whether you will stay on-site or off-site. The Disney bubble is a must for some, but there are a number of advantages to staying off-site, especially with 3 kids or if you are on a budget.
Advantages of staying off-site:
- More space for your money, including separate spaces. With small children we really appreciated not having to tip-toe around the nappers or turn out the lights in the whole space when it was the kids' bedtime.
- Availability of accommodations with kitchens. Good for budget maintenance, feeding kids who may not all be ready to eat at the same time all the time due to park schedules, or if you just find eating out more than once a day with your kids to be the exact opposite of a vacation (this was us). You can get villas with kitchens on-site at WDW, but they cost from 5-10x more than the same room off-site.
- Ability to leave the Disney bubble when your kids are massively overstimulated. This is one of those Your Mileage May Vary things, but for all that many will tell you how wonderful it is to be wrapped in the Disney bubble, there are people and families who find that the theming and crowds of the resorts is still just too much after a busy day in the parks, and leaving Disney entirely for something that resembles "normal" life allows them to come back more refreshed to the parks the next day.
Advantages of staying on-site:
- Access to EMH
- Access to FP+ booking at an earlier point
- Easier to split up if say one parent will take toddler back to room to nap while the other parent stays in park with kiddos -- no one is ever left without transportation
- Can avoid renting a car if you don't want to
A compromise is the hotels in Downtown Disney / Disney Springs -- many of these offer suite accommodations at a comparable or less price to Disney values and moderates. You won't have a full kitchen, but you may have an in-room fridge or kitchenette with microwave, and separate (from your rental car) transportation to the parks won't be optimal but it will exist. If separate spaces is appealing to you but you think you won't use a kitchen extensively and will split the party only rarely but it might happen, this might be the way to go. If you do decide to go "completely off-site", I would recommend looking into renting a condo, there are many available in the $100 / night range that will give you two bedrooms and a kitchen. For the same price as a hotel room, why wouldn't you want that?
Side-by-side with the decision on whether to stay on-site, you should consider your touring style which may be quite, quite different with kids than it was without kids. Things like breaking to take mid-day naps / rest, splitting up the party, desirability of having a rental car, etc. will help you make this decision. For instance, if you think you won't really plan to use EMH, then that is no longer an advantage of staying on-site. If you definitely don't want to rent a car, then that is an advantage of staying on-site. If you want to be flexible about sit-down meals because you don't know how it will go each day and you plan to eat them before or after a mid-day break, that would be an advantage of staying off-site (or at least getting a rental car) because it gives you access to a wide variety of off-site restaurants that you can reliably walk into without a reservation and get seated.
These same decisions about touring style will also give you some insight into what to plan across the board, for instance if you decide to stay off-site ADRs may be less of a priority except for one or two that you pick out as "must-dos". If you're only making 1-2 in-park ADRs, now you can do away with the overhead of picking your park days 180 days out so that your ADRs don't conflict with your planned parks. Conversely, if you decide to stay in the parks all day and leave before dinner, now you might really want to make sure you have a mid-day ADR each day for a chance to sit for a while and get out of the heat. This is especially true if you get the dining plan, as many kids (although maybe not yours, only you know them) will often do better with a TS meal mid-day when they have more stamina and a less-demanding-of-patience-and-manners food court meal in the evening when they are tired than the other way around.
Anyway, I've run on and on but I wanted to give a good justification for why I think on/off-site and overall touring style are the two things to plan first. But basically it boils down to: once you've decided these two things, many of the other decisions about what to plan / prioritize flow directly from them.
