Select count without join
This would probably need to be a UNION query, as in:

SELECT 'a'
  , COUNT(a.time)
FROM
  tbl1 AS a
WHERE
  a.time BETWEEN '2018-03-07 18:32:55' AND '2018-03-07 20:46:55' 

UNION ALL

SELECT 'b'
  , COUNT(b.time)
FROM
  tbl2 AS b
WHERE
  b.time BETWEEN '2018-03-07 18:32:55' AND '2018-03-07 20:46:55' 

UNION ALL

SELECT 'c'
  , COUNT(c.time)
FROM
  tbl3 AS c
WHERE
  c.time BETWEEN '2018-03-07 18:32:55' AND '2018-03-07 20:46:55' 


The output would look like:

╔══════╦══════╗
║ col1 ║ col2 ║
╠══════╬══════╣
║ a    ║   10 ║
║ b    ║    3 ║
║ c    ║   14 ║
╚══════╩══════╝