Custom field containing date in mini loop

I am a new WP user so this is a dumb question:

I have a series of posts of activities. I have created a custom field in those posts called activity_date. Would I input this date, for example, as 20170328

I’d like the mini loop to display the next 3 activities after today. I think I need to put a bit of code in the Custom Fields box.

How do I do this?

Thanks