The simplest way is with "copy /b 1.pcl+2.pcl+3.pcl all.pcl". If you need to peak inside each PCL file to determine the printstream order, then you'll need PCLTool SDK - Option V. You would need to use a PCLXForm .tpt script program to set-up the rules to determine which PCL file is to be placed in which order, etc.
PCLXForm also an overlay, insert and extract images, pages or text during the PCL streamediting process. And, it can convert the result into PDF, TIF, PNG, etc.