When we do a 3 nighter I strategically meal plan for minimal faff and left overs/stuff to bring back. We generally do a click and collect grocery order from a nearby/en route supermarket. We generally eat breakfasts and evening meals at the caravan and lunches out (DC is autistic which makes eating out tricky)
E.g. when we went recently I took...
Homemade curry for night 1 - cooked in advance, taken out of the freezer that morning. Was defrosted by the time we got there. Added microwave rice pouches and accompaniments.
Night 2 - homemade lasagne, again made in advance and frozen. Take out before we leave, acts as a nice ice block for any chilled stuff whilst travelling, and defrost slowly in the fridge ready for dinner on evening #2. Usually add some garlic bread too.
Night 3 - either a takeaway or something simple that doesn't result in bits left over e.g. a pack of 6 burgers and 6 rolls and we'll have 2 each.
My other tips...
Be prepared for no/poor 4G/wifi. We take books, board games & DVDs (the booking info should tell you what there is facilities/equipment wise)
Take a decent sized mug for your morning brew. The ones they provide are usually tiny.
Small amount of washing up liquid (I use a little toiletry travel bottle), a sponge, few dishwasher tablets, tea towel etc.
Bin bags - we have a bin bag that we fill with dirty washing. No need to sort clean from dirty when we get back.
Loose change for on site arcade if there is one.