I often find that in such cases it's helpful to consult a kanji dictionary and other words with the kanji involved to get a feel of the range of meanings.
関わる [kakawaru] be concerned in, have to do with
係 [kakari] 1) charge, duty, concern; 2) person in charge
関係 [kankei] 1) relationship, relation, connection, reference; ~ga aru, ~suru be related to smth., have a connection, concern; ~naku regardless of smth.; 2) involvement, participation, concernment; ~suru be involved, participate; 3) influence; ~suru have influence on smth.; 4) connection, sexual relations
連れる [tsureru] take smb. along
連続 [renzoku] 1) continuity, succession, series; ~suru continue, last; in compounds continuous; 2) continuously, without a break
連絡 [renraku] contact; connection; liaison; communication; ~suru
contact; connect; communicate
(definitions from the JiShop dictionary)
From which we can infer:
- 関する is probably similar in meaning to 関わる, i.e. it hints that something is connected or relates to the subject being mentioned.
- 関係 is possibly used to describe human relations or similar.
- 関連 seems to hint at some kind of connection or link between the two subjects.
Of course, in practice it's a bit more nuanced as naruto describes, for example in the vast majority of cases 関する is used as a set expression ～に関する～ and 関係 is not limited to human relations.
Another good option is to do a Web search for "X と Y の違い". There are multiple sites and blogs devoted to such topics and even though most of them are in Japanese you should be able to get a gist of it with some work (and practice your Japanese at the same time ;) For example, from this page:
関連 shows that "those people or things themselves have a direct
関係 shows that "those people or things have a relation, even if
The main difference is "direct or indirect" point of view.
There are also good examples on the page which show when the specific word should be used or not.