Get future multi-day event entries with custom date field
I think the simplest solution would be to make the eventEndDate field required even for single day events. Then you could do something like this:

{% for event in craft.entries({
  section:'events', 
  eventEndDate:'&gt;= '~now,
  limit: 3, 
  order:'startDate'
}) %}


Given you have 3 events:


Starts Monday, ends Friday.
Starts Wednesday, ends Thursday.
Starts Thursday, ends Thursday.


On Wednesday, you'll still see those events in order (1,2,3).