vegetable plants start well, then wilt
Thanks for your question about your question about vegetables wilting. There are a variety of causes of vegetables not reaching maturity.
Tomatoes are subject to tomato late blight which effects both the leaves, and fruits may also be infected. Here's a link that explains what it is and how to control it. If, on the other hand, you see that the tomato fruits themselves have black spots and do not develop, they probably have blossom end rot, which means the soil is to acid (low pH), and the plant can take up calcium You'll need to test and adjust your soil for next year.
Squash and cucumbers are members of a different family than tomatoes, and they develop their own problems. Among other diseases, they develop down mildew, which is caused by a fungus, and you'll see gray spots on the leaves, they'll turn brown. Depending on when this happens in the growing season, the fruits may be able to develop normally. Here's a link to the disease control handbook on downy mildew.
Since there are at least 3 other common fungi that can attack squash and cucumbers, including fusarium crown and root rot and fusasium wilt. Without having a photo of your plants, including the roots, it's difficult to ascertain which your plants were subjected to. Hopefully, reading these articles (especially the 'controls' sections) will give you some help with avoiding the problems next year.