SQLite: Count participants of questions

I have 3 tables. I currently run following query which does not work.

SELECT
  a._id,
  a.title,
  COUNT(DISTINCT user_id)
AS
  number_of_participants
FROM
  a, b, c
WHERE 
  a._id=b.fk AND
  b._id=c.fk

The field user_id exists only in table c.

Explanation what I want: Table a contains questions, while table b holds the answers with reference fk to the question. And I want to count how many users already have voted for every question. So table c holds the votes.

EDIT: My database scheme:

Table a:
_id integer,
title text

Table b:
_id integer,
title text,
fk integer references a._id

Table c:
user_id text,
name text,
fk integer references b._id