GROUP_CONCAT with limit

One somewhat hacky way to do it is to post-process the result of GROUP_CONCAT:

substring_index(group_concat(s.title SEPARATOR ','), ',', 3) as skills

Of course this assumes that your skill names don't contain commas and that their amount is reasonably small.

fiddle

A feature request for GROUP_CONCAT to support an explicit LIMIT clause is unfortunately still not resolved.

UPDATE: As user Strawberry points out, the table player_skills should have the tuple (player_id, skill_id) as its primary key, otherwise the schema allows for the same skill to be assigned to a player multiple times, in which case group_concat would not work as expected.

It is possible if you are using MariaDB 10.3.3+:

Support for LIMIT clause in GROUP_CONCAT() (MDEV-11297)

SELECT p.id,  
   GROUP_CONCAT(s.title ORDER BY title  SEPARATOR ', ' LIMIT 3) as skills
FROM player p
LEFT JOIN player_skills ps ON ps.player_id = p.id
LEFT JOIN skill s ON s.id = ps.skill_id
WHERE ps.value > 2
GROUP BY p.id 
ORDER BY s.id;

db<>fiddle demo

Increase GROUP_CONCAT function length using GLOBAL group_concat_max_len GROUP_CONCAT() maximum length is 1024 characters.
What you can do is to set the GLOBAL group_concat_max_len in mysql

SET GLOBAL group_concat_max_len = 1000000;

Try this and it will work for sure.

There is a much cleaner solution. Wrap it inside another SELECT statement.

SELECT GROUP_CONCAT(id) FROM (
    SELECT DISTINCT id FROM people LIMIT 4
) AS ids;

/* Result 134756,134754,134751,134750 */