Peace be upon you
In the meanwhile while Brother Joseph answers, I would like to give my response to it as well.
The root word is Fa-Ta-Nun ف ت ن
Meaning - to try or prove, persecute, burn, assay, put into affliction, distress and hardship, slaughter, cause to err, seduce from faith by any means, mislead, sow dissension or difference of opinion, mischief, put in confusion, punish, give reply or excuse, tempt, lead to temptation, make an attempt upon, seduce. fitnatun - persecution, trial, probation, burning, assaying, reply, excuse, war, means whereby the condition of a person is evinced in respect of good or evil, hardship, punishment, answer, temptation, burning with fire.
The places the word fitna is used as a verb in the Quran: [1]
perf. act. 6:53, 20:40, 20:85, 29:3, 38:24, 38:34, 44:17, 57:14, 85:10
impf. act. 4:101, 5:49, 7:27, 9:49, 10:83, 17:73, 20:131, 72:17
perf. pass. 16:110, 20:90
impf. pass. 9:126, 27:47, 29:2, 51:13
n.vb. 20:40
pcple. act. 37:162
pcple. pass. 68:6
Meaning trial, to put in hardship, being tempted, etc.
The places the word fitna is used as a noun in the Quran:
2:102, 2:191, 2:193, 2:217, 3:7, 4:91, 5:41, 5:71, 6:23, 7:155, 8:25, 8:28, 8:39, 8:73, 9:47, 9:48, 9:49, 10:85, 17:60, 21:35, 21:111, 22:11, 22:53, 24:63, 25:20, 29:10, 33:14, 37:63, 39:49, 51:14, 54:27, 60:5, 64:15, 74:31
The places you cited 8:39, 2:191 and 2:193 are all using the noun form.
The word used in 8:39 فِتْنَةٌ has a many meanings, but as God said we should always follow the best meaning.
039:018
"Those who listen to the Word (the Quran) and follow the best meaning in it / best of it (Arabic: fayattabi'una ahsanahu) those are the ones whom God has guided and those are the one's endowed with understanding (Arabic: Albabi)So the meaning
"it signifies also Punishment, castigation, or chastisement. (T, M, Ḳ.) And Slaughter: (T:) and civil war, or conflict occurring among people: (M:) and slaughter, and war, and faction, or sedition" - From Edward Lane's Lexicon
You can find the complete meaning of the word from here
http://lexicon.quranic-research.net/data/20_f/023_ftn.htmlYou can also confirm the meaning of the word by going through the lexicons available here
http://ejtaal.net/aa/#q=ftnIf I was wrong, please correct me
hope that somewhat answers the question in the meanwhile.
[1] -
http://www.studyquran.co.uk/PRLonline.htm