Make sure you have updated WC3 to the lattest non beta version available on battlenet and also that you have updated your graphic drivers to the lattest version. No one can help you before you have done this.
If these are normal WC3 models (not custom imported) and you are running on vista or windows 7, you may need to run the program as an administrator. If that does not work, reinstall the game (that part of the data file may be curropted). Try running the game at lower visual settings, this may bypass errors like this at the cost of play quality. If none of the above works, then it is a physical hardware problem and you should eithor contact Blizzard or check if such problems occur playing another game.
If the model is a custom imported model then it may have become curropted at some point so try redownloading it and reimporting it. If it still crashes the editor, contact the maker to check you are not importing it wrongly or to make sure that it works.
Crashes like this are often related to your graphic card. If your graphic card somehow struggles to run WC3 (which no modern desktop cards should do), it may cause crashes. Posting your system specifications could help diagnose this problems.