[Bibio]

@ ausvette  try this:

<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="mkv" player="dsplayer">
   <rule filetypes="iso" player="MPC">
  <rule filename=".*3D.*" player="MPC"/>
   </rule>
 </rule>
 </rules>
</playercorefactory>

from what i can see in your other code you have forgot to add the DOT before the * doing this will pick up any wildcard filename with or without 3D.
the above code should force dsplayer to play all mkv files except ones that meet the criteria of 3D in their filename.
all files with the extension iso will be played with MPC.

btw. kodi-dsplayerx64 is a bit buggy. been there tried that.

 Thanks Bibio.

I tried that.

Result was dsplayer was default player for all mkvs - whether they matched the rule or not.
Videoplayer was default for iso's
Mpc was listed as a player I could choose manually from the menu 

 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>