You don't need table3 this way.
it should be
drugs2se
---------------------
d_id | se_id
---------------------
1 | 1
1 | 2
3 | 2
3 | 3
Then you can get desired results with a query like this
SELECT d.name, group_concat(se.name) as effects
FROM drugs d, drugs2se dse, side_effects se
WHERE d.id=d_id AND se_id=se.id
GROUP BY (d.id)