The Hidden Replica Set Members page of the MongoDB docs (version 3.4) states that
If you stop a voting hidden member, ensure that the set has an active majority or the primary will step down.
What is an active majority?
What if a hidden member of a three-member replica set is stopped? Do the two remaining members constitute an active majority?
What if a hidden member of a three-member replica set is stopped? Do the two remaining members constitute an active majority?
The replica-set has three members - one of them hidden, all are eligible for voting and the remaining two are eligible to become primary. If the hidden member steps down, the remaining members do constitute an active majority. The replica-set will be operational.
In a three member replica-set it requires two members to be operational, i.e., to function as a replica-set, perform replication of data and accept the reads and writes from clients. In a three (odd) member replica-set the majority is 2 = 3/2 + 1.