You have to use parentheses to separate each join. The query would be like this:
SELECT A.userid, A.date, A.source, B.completed_team, C.send, C.open, C.click
from (( B
left join A on B.userid = A.userid )
left join C on A.userid = C.userid);
The output of the above code will be:
userid date source completed_team send open click
1010 4/20/2020 updown Tokyo 20 12 8
1012 3/20/2020 leftdown New York 12 8 6