Milkshape is free and great.
3DS Max is a waste of money. Unless you bought it 5 years ago, you'll never find a copy of version 5. The present version of the Max does not support Art Tools, so you can't make wc3 models with it anyway.
Milkshape however, is free, awesome, and it supports WC3 so go for it and save yourself a lot of money.