To use eCart 3, you will need to use Dreamweaver 8 or MX 2004.
If you are using Dreamweaver CS3 or above, you will need to use the latest version of eCart which is version 5.
eCart 3 sites are not compatible with eCart 5. If you upgrade an eCart 3 site to eCart 5, you will need to recreate any cart displays, and you may need to recreate your charges or discounts as well as the checkout process. This is due to changes in the eCart code that where made to in version 4 to make taxes and shipping rules more robust.