Hi,
Thanks for using our plugins. I can try to help out here, but we are limited in supporting customizations per our terms and conditions.
Anyway, the queries that are there are designed to show only upcoming events and there is no feature to always show 3 events.
It would take an modification to do that as it would have to run two queries to get both upcoming and past events and compare the output to see if the past events are needed. There is nothing in place to do that.
We can try to help you get started on a customization like that, but not able to support features beyond what we ship with.
If you think that is something you would like as a feature I encourage you to visit our User Voice Page and either upvote an existing request or make a new request there.
http://tribe.uservoice.com/
Let me know if you have any follow up questions.
Thanks