It depends on what OS you are running and a little on what edition of wcIII you have. (Im not sure but I think the classics edition is fit for Win95 enviroment)
Anyway there are 3 basic progs. out there for what you are looking for.
I think all are available on this site somwhere in the downlaods section.
-MoSlo - only ms-dos games my friend.
-Turbo - win enviro compatible slows down comp by percentage you set.
-cpuKiller - Probably best one this is only the demo though and the prog. will only let you play for 20 min. You have to go to the programmers website and pay him $25 for the reg. code. Despite this, this prog. is nice for 2 reasons - runs pretty well in windows. And even better is set up so you can launch a game from the prog. and limit just the game.
Good luck.........but Let me give a little bit of advice my friend.
I recently decided to go back and play all the old WC games that i Loved from my childhood. (This was about 3 Months ago) And so since the only one I still had laying around was WC4 I went on the internet and through ebay and other such places I was able to get everything that I wanted specifically WCIII and WCP. Now i wanted to start the series from the begining. Well of course the first game I get in the mail is WCP and then much much and I mean much later I finally get WCIII after waiting for months trying not to play WC4 and WCP(which was hell), well I go to install the game and my friggin' brand spankin' new microsoft sidewinder precsion 2 joystick won't work with the game. I spend a couple more days playing around with it and talking to people on the boards and in mirc and nada.... so finally I get the bright idea to go down into my garage (which is a pit) and attempt to find my 3DO with WCIII which is how I first got turned onto the game way back when I was young. Well I had thought for some reason that I wouldn't be able to find it and that I was missing one the disks. Well guess what not only did I find it, not only were all the disks there, but i even was able to hook it up to my tv (this was a amazing luck)with the cord that went to it that I just happend to still have in an old box of game connection cords. After that I hooked up the thrustmaster i had bought when i got the 3DO and went to town.
My point is I waited 3 damn months for that stupid game to show up and then when it didn' work I just went and got my 3DO which worked better anyway cause it's got more of the video than the computer version. And because sometimes especially with older games computers can be too much of a hurdle to have to deal with.
If I were you and if you really want to play that game to your hearts content either try to find a 3DO and the WCIII game(they have gotta be cheap cause 3DO went out of style so fast it was ridiculous, the system was one of the biggest jokes around.) Or if you game on systems other than computer you may have a playstation and if you do all you have to do is go out and get the WCIII game.
Well thought I would just ramble a little either way good luck hope this info helps.
Go into DOS and run Mo'Slo on COMMAND.COM (you'll find it in C:\). That'll start a slowed-down DOS session, wherein everything you do will be slowed down. Then, run the install program as you normally would, and when you get back to DOS, type EXIT to get out of the slowed-down session.