1 Folder inside got 1 MKV file 3 SRT different language subtitle, I unable to switch 3 different subtitle. Please help..

Mux the subs into the MKV file with MKVToolnix (MKVMerge)

just name the three srt files exactly the same as the movie files name,
then add the language at the ende of the filename start with a dot then the language name:
example

movie: Test.mkv
srt : Test.ned.srt
Test.eng.srt
Test.ger.srt

plus in the setup menu you need under misc switch on the subtitle ,