I have 2 DS and 1 DSD. Holidays have always been interesting..... My 2 have their own additional needs; DSD is used to getting his own way all the time. Taking this into account plus having a child that has sensory difficulties and therefore had a restricted diet plus a DSS that has never been encouraged to try new things and only eats the same plain food all the time my advice would be this:-
Do not make your first holiday the place to have this battle. If all DSD will eat is chicken nuggets and chips (for example) then fine - 4 days of this wont hurt her. Just do your research about where you're going and make sure there is something on the menu she would usually eat.
Also, agree with DP in advance that if a child (any child, not just DSD) behaves unacceptably during a mealtime/day out etc, their parent will remove them so everyone else can enjoy their meal/day out.
Consider taking 2 cars (if you both drive) so if 1 child is misbehaving the rest can still go out - DSD (and DP) learnt the hard way once when he was kicking off about going out for dinner so I left DP at the chalet with bread ham and cheese for sandwiches for them both and took my kids out for a meal (I have also stayed behind with one of my kids before and DP taken DSD and my other when they've misbehaved).
Dont expect it to all be plain sailing - it is going to be a lot of adjustment! Try to keep things pretty relaxed, easy going and not too regimented.
As there's 3 kids, maybe allow each child a choice of activity / meal option each day eg day 1 A chooses morning activity, B chooses afternoon, C choose dinner venue - obv all these activities are from a pre agreed list you and DP have chosen. This way, no one can complain they never get what they want (expect you and DP of course but thats the joys of kids isn't it? lol).
I'm sure if you and DP discuss expectations with each other before and ler the kids know how things are going to work it will be fine - hope you have a lovely break!