thrashosds: change min_in from 2 -> 3

See #7171. In rare cases CRUSH can't handle it when only 2/6 of
the OSDs are marked in.  Avoid those situations for now.

Signed-off-by: Sage Weil <sage@inktank.com>
Merge pull request #174 from ceph/wip-7171

thrashosds: change min_in from 2 -> 3