Rachel explained it pretty well.

Eventing consists of 3 English disciplines, they are referred to as "phases", Eventing can be either take part over four days or be a one day event.
Three day events are the ones that run over four days (usually), dressage usually takes part on the first two days, cross country the third day, and show jumping on the fourth day.
It is set in that order to test the horse's obedience and stamina (having to be controlled and precise during dressage, then needing to open and run for cross country, back to show jumping for control and precision while also testing the agility.)

A one day event is usually for lower levels, you won't really get a beginner going to a 3 day event (in real life, sims is different of course). One day events contain all 3 phases as the 3 day event, but it only runs for one day as the name suggests.

I would highly recommend going on youtube and looking up 3 day events, one in particular, Badminton Horse Trials, it is a 3 day event and one of the most well-known top eventing courses. There are videos on their website as well as showing you the cross country course etc. www.badminton-horse.co.uk/
--

Attire here depends on who is hosting it and their decisions, each host comes from a different place where there are different attire rules or "norms" irl, so I would do a little bit of research on attire, but first read what the show host has said in their attire rules.

Of course each host will also have their own set of rules in regards to jump height and such, so that's another thing to research into but pay close attention to the host's rules.

--
0
Eventing is something my stable focuses on (and what I hope to do with my IRL youngster when she's old enough), and I am an English rider, and trainee instructor IRL so I know a lot about the English disciplines etc, if you need help let me know, my PM is always open and I'll do my best to help you.