Kody, you are allowed to host multiple disciplines at the same time.
Cross Country is not a discipline in its own right however, only a phase of eventing. Pretty sure it wouldn't be practiced in the snow either as that could be quite dangerous. In fact, in RL most cross country courses close during the winter because of that. Eventing shows at that time will either just host only two phases (dressage and show jumping), or if they have the amenities host an indoor cross country /show jumping combo phase.
If you want to host the show im thinking you want to host, then I would go with a discipline like Endurance. It will allow people from all different backgrounds to partake (may it be English or Western). I know it's in a different field, but keep in mind that everyone can enter a local show, regardless of their usual discipline.
Also, I will add that the trail discipline in the Western field was more hinting at the courses practiced indoors... not actual endurance.
Another discipline you may want to consider would be TREC as it does have a trail ride component (however it is not supposed to last more than 2 hours... so definitely not 3 days)