Don't think there IS a minimum... didn't see anything about it in the rules.
Only minimums I see are the recommendation of 25 kids in fb and eligibility for a District Title.
With the new playoff format, I think a Div. 1 team could go 1-2 and get in.

About being obligated... Rule 68-7-1 says, in part...
"A school which refuses to play the minimum number of district opponents shall be ineligible for the district championship or for any consideration for a play-off berth in any championship series"
So, there are a few ways to get around it if the schools don't close.
The easiest is for the District to vote on what the minimum number shall be (I think they probably need to do that, anyway).
They can also allow the "readmitted" schools to "designate" certain games to count in the district standings (assuming, of course, they can find games. I'm sure they could get at least 6... play the other 2 twice, and get 1 existing game somewhere broken up so they could play 1 game against each of those 2 teams).
As a last resort, the Region could add a 5th District which consists of just those 3 schools.