I now have to create four or five different flight plans for the same flight before I fly: one for PF3, one for X-Plane, one for Little NavMap (containing all the SID/STAR information) and often yet another one for my aircraft's FMC - a number of planes in XP need their own format or adaptation to work properly; I have at least four which need their own adaptation. I guess it's all a labour of love in the end though!
It can be done with a few clicks within LittleNavMap.
Here is what I do:
1. Create a flightplan with Simbrief and export it to the .pln format.
2. Load that plan into LittleNavMap and if necessary adjust it the way you really want to fly it (including SID and STAR if you always know the weather beforehand - isn't that boring btw?
3. Export that plan to the .pln format (resp. the X-Plane flightplan format) and to the format for your addon aircraft (LittleNavMap should be supporting most of these by now).
4. Then adjust that plan for the needs of PF3 by adding or removing waypoints if necessary (make sure you reach all PF3 waypoints with your "real" flightplan).
5. Finally export that plan to the .pln format, renaming it ..._PF3.pln.
So you have
a) .pln / X-Plane format for use with the sim and LittleNavMap
b) addon aircraft format for use with the addon aircraft's FMC
c) .pln format for PF3 for use with PF3.