Thanks, Baozhong Lee, for your posting. Surprisingly, the adjective "content" may in fact be used attributively, i.e. before a noun. It's not as common as "contented," but it is used. Here are some examples from Google:
--I live a rather content life with my wife and our cat, and still find time for social activities and other hobbies that I enjoy.
--It has been a long time since I had this content feeling about AMAZING SPIDER-MAN.
--Noah is so sweet and happy. She isn't always laughing....she is just happy in a content way . As long as her basic needs are met, she doesn't demand attention.
I did not, however, find any occurrences of "put[s] on a content look," as in Sentence 1, although I found some instances of "a content look" without "put on."
There's also more to say about the participial adjective "contented." "Contented" may occur predicatively, i.e. as a subject complement. Thus both "content" and "contented" can be used in Sentence 2:
2. He is content/contented with his marks at the finals.
Here are a few examples of predicative use of "contented" from Google:
--This pool of talent is quite contented to be in India, and bubbles with enthusiasm driven by a sense of continuous challenge.
--As I looked across the crowd, everyone looked contented just watching some good bands and talking to their friends.
--Yet how often do any of us truly feel contented ? In a nation overflowing with material affluence, how rarely do we appreciate what we have?
Sentence 3 has a passive verb, and needs a past participle, not a participial adjective. The only correct form is
3. He is quite easily contented by a small thing