As Cambridge Dictionary says, "proper" means: "real, satisfactory, suitable, or correct". Also, it is said that, "suitable" means: "acceptable or right for someone or something". An finally, "appropriate" means: "suitable or right for a particular situation or occasion".
As you see, they have lots of semantic overlaps which makes a person who lacks the requisite intuition fall into doubt when it comes to use each case. This is why I decided to provide you with some examples to discover how and when to use each case. I would appreciate it if you kindly let me know about the wrong choices for each case. Here are my examples:
- Such movies are not ........... for children.
a. suitable
b. appropriate
c. proper
- Two out of five people lack a .......... job in that country.
a. suitable
b. appropriate
c. proper
- Such small studios are ............ for young couples who have no children.
a. suitable
b. appropriate
c. proper
- I can't come to the ceremony tomorrow! I don't have any .............. clothes.
a. suitable
b. appropriate
c. proper
- It is not a/an .............. time to ask her about this matter.
a. suitable
b. appropriate
c. proper
- We will take .......... actions when it comes to.
a. suitable
b. appropriate
c. proper
- They dont consider him a/an ............. husband for their daughter.
a. suitable
b. appropriate
c. proper
Please note that, to me as a non native, all choicea work for all cases above.
I have already visited the similar thread on the forum.
Some useful links:
https://forum.wordreference.com/threads/fitting-suitable-appropiate-proper.2313993/
https://hinative.com/en-US/questions/3204687
https://www.englishforums.com/English/SuitableProperAppropriate/bbcnwp/post.htm
https://www.italki.com/question/191255
https://www.usingenglish.com/forum/threads/96774-what-is-difference-between-approperiate-and-proper
http://www.bbc.co.uk/worldservice/learningenglish/grammar/learnit/learnitv108.shtml
https://www.quora.com/What-is-the-difference-between-proper-apt-and-appropriate