generaly the horse stuff is pretty obvious, but it is the rider stuff that stuffs one. You dont need to be stressed by being hungry short on sleep and dehydrated ,and concerned that you smell like a skunk . In the army we learned how to deal with the first three (but we still smelled like skunks after 14 days in the field). For me the single worst failure is coms,(communications) If your mobile phone runs out of charge you are going to suffer. . A cool box is nice , and treat yourself to a few luxurys cos its not going to be all roses. And remember.with adequate coms ,any problem can be dealt with given sufficient warning.
Blimey - it's a 3DE not a reconnaissance expedition in Afghanistan! Will you be camping in a tent or a lorry? In my experience you need a comfy, dry, warm bed and then all will be well. Take an airbed (and something to blow it up with), and a duvet and pillow rather than a sleeping bag. A sheet to cover the airbed will make it more comfortable too.
Lots of wet wipes, a torch, batteries, more spare sets of clothes than you think you need in case it tips down and you get soaked on multiple occasions, one of those magic superabsorbent towels, food and drink (mostly soft, some alcoholic). Possibly a disposable BBQ although they are not popular in many places.
Sleeping in my lorry, Can't do tents lol. For food think we are going to find a local pub for evening meals (don't have a fridge to store food) plus it saves cooking after a hard day ;-)
Oh think there will be a little bit of alcohol going round.
I had thought about extra cloths for competing, totally forgot about extra for everyday cloths. Mind you i'm hoping the weather will be good to us and we don't see any rain while we are there