[Bibio]

 remove <rule filetypes="mkv" player="dsplayer"> from the code and try that. i put this in as a test... but it was worth trying just in case.
dsplayer is the default player in kodi-ds so should play all files except those in the code playercorefactory code rules.

also remove one of the </rule> at the end

pay close attention to <rule filename=".*3D.*" player="MPC"/> the filename rule must start with a DOT then *3D.*
if you dont add the DOT then kodi will play all finenames with mpc as you are starting with a * (wildcard) which is all your filenames.

your code should now be:

<playercorefactory>
 <players>
<player name="MPC" type="ExternalPlayer" audio="false" video="true">
       <filename>C:\Users\Dave\AppData\Roaming\Kodi\userdata\Play3D.bat</filename>
       <args>"{1}"</args>
       <hidekodi>true</hidekodi>
       <hideconsole>true</hideconsole>
       <forceontop>false</forceontop>
       <playcountminimumtime>1140</playcountminimumtime>
    </player>
    </players>
 <rules action="prepend">
   <rule filetypes="iso" player="MPC">
  <rule filename=".*3D.*" player="MPC"/>
 </rule>
 </rules>
</playercorefactory> 

Thanks so much for your help and persistence with this.

With this, kodi is playing everything with dsplayer except the iso files which are playing with mpc.

Nearly all my 3d movies are iso's so that's great progress.

My 3d mkv files are still playing with dsplayer. My filenames are of the form "movie title 3D.mkv" 

Do I need to rename these to "movie title.3D.mkv" to match the rule?

Thanks!  

no dont add a DOT to your movie filenames. your not trying to match the filenames your making the code match the filenames.

the code ".*3D.*" is saying anything contained after the DOT is a wildcard but must match 3D as well, the .* at the end is saying match any wildcard e.g. mkv/iso/avi but must also include the filename with 3D as the whole quote is wrapped in an expression " ". if you dont add the DOT at the beginning then it will say match any name with or without 3D as a wildcard.

since all your "movie title 3D." are mkv then it will only pick up the .mkv
you could also try removing the last * (wildcard) and specify the extension e.g. ".*3D.mkv" this should force the extension.

try adding another </rule> above or below the one already at the bottom. if that dont work then remove the </rule> that is there and try again.

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BTW. if you added 3D to all the iso movie filenames then just having <rule filename=".*3D.*" player="MPC"/> is all you would need in the code as it would pick up any filename with 3D and any extension e.g. iso, mkv.
you would then delete <rule filetypes="iso" player="MPC"> from the code.