Hello there,I will try to explain them to every little pieces,so please bear with me: I completely agree with @invisibleteacup on their differences,and I would like to point out some details with their usage:
Fikir is the old version of düşünce,so most of the new books uses düşünce instead of fikir,but since the fikir is the old version,there are some usages that will be good to use with fikir instead of düşünce.It will make you sound more native and natural:
Bilgi sahibi olmadan fikir sahibi olma (Do not have any opinion before having any knowledge)
Bu konudaki fikrin ne?(What is your opinion?,you should use fikir because it is general and natural usage,if you want to use the verb düşünmek,you might say sen ne düşünüyorsun instead of of senin düşüncen ne?)
Enine boyuna bunu düşün taşın.(Think about this throughly)
Akıllı düşünene kadar deli köprüyü geçer.(The crazy would cross the bridge before the wise develops an opinion-on the subject whether or not to cross it)
These are what comes to my mind for now,hope I added some information that would be useful!Keep going,cheers!
I noticed that "görüş", "fikir" and "düşunce" mean "opinion" and/or "idea". Is there any difference or are there completely interchangable?
Görüş/fikir is one of the many synonym pairs caused by Turkish language reform: fikir, being of Arabic origin, was replaced with görüş, a new word from Turkic roots. All three are basically synonyms. I might say that düşunce is a little closer to "thought/idea" and görüş to "opinion/view," but I don't know if everyone would agree
No. Think is a verb. The noun form is 'thought' (which is also the past tense). You could say 'what is your thought' and it would just about make sense, but it sounds strange. Stick to 'what is your opinion' or maybe 'what is your idea' although that has a different meaning. 'What is your opinion about x' means 'what do you think about x' and 'what is your idea' means 'what do you suggest'. I don't know whether the Turkish sentence has both meanings.