Try it like this, I have no schema to test it myself but I feel like it should work(or something like this)
SELECT * FROM questions, connections
LEFT JOIN answers ON (questions.id = answers.id AND
connections.username2 = answers.username)
where connections.username1 = 'mikha';
eventually like this
SELECT * FROM questions
LEFT JOIN answers ON (questions.id = answers.id)
LEFT JOIN connections ON (connections.username2 = answers.username)
where connections.username1 = 'mikha';
EDIT: I found this in documentation
Example:
CREATE TABLE t1 (i1 INT); CREATE TABLE t2 (i2 INT); CREATE TABLE t3 (i3 INT); SELECT * FROM t1 JOIN t2 ON (i1 = i3) JOIN t3;
Previously, the SELECT statement was legal. Now the statement fails with an Unknown >column 'i3' in 'on clause' error because i3 is a column in t3, which is not an operand of >the ON clause. The statement should be rewritten as follows:
SELECT * FROM t1 JOIN t2 JOIN t3 ON (i1 = i3);
So for Your case it may be
SELECT * FROM questions
LEFT JOIN connections
LEFT JOIN answers ON (connections.username1 = 'mikha' AND questions.id = answers.id AND
connections.username2 = answers.username)