It depends on how closely each of the manufacturers you have mentioned adhere to the standard. The IEC have no interest themselves in policing the standard with the result that an organisation called PLCOpen has taken on the task of defining test procedures for evaluating programming software. They award various certificates of compliance for the different IEC-61131-3 languages.
However, two manufacturers could create language editors which both meet all the requirements of the standard, but then add in some extra features or instructions not defined by the standard. If you stick to using the basic set of instructions you may be able to recompile a source file to be used in either package. But as soon as you stray in to the 'specials' there may be no corresponding instruction in the other's language to handle this.
There is certainly no possibility whatever of taking a Mitsubishi program in its entirety and loading it in to Siemens or Rockwell PLCs. At best you may be able to take certain code elements or block structures and recompile them within Siemens' or Rockwell's environment.
Regards
Ken