if i play those command :
sync echo 3 > /proc/sys/vm/drop_caches
Will the server be force to read the correct file on disk until i put 0 again ?
Or this work only one time, and if i want to clear cache again, i've to put 0 and 3 again ?
No - that just drops the existing caches. As soon as you start reading files, linux will start caching them again.
There shouldn't be a scenario in which linux reads "old" data from a disk cache though, if that's what you are worried about.
You don't need that kind of wizardy. Linux will cache files it reads, but a write operation invalidates the cache entry for that file and it will be read again from disk during next read access.