Advice needed On Sitemap.XML Setup

Im in the process of creating a new, fairly large site which is layed out as follows;

at the root I have my main site, which is HTML
in a subfolder called BLOG I have a wordpress Blog
in a subfolder called SALES I have a Magento setup.

Can anyone advise how I should be setting up my sitemap.xml? should I have 3 sitemaps, or just one at the root.

What tool(s) should I use to create the sitemap, is there anything which can do it automatically as a cron job?

Many thanks guys.

I wud do one sitemap for all sections. Creating more than one is not bad but it is a bit weird in the eyes of the search engines. Creating 3 of them might indicate to the se that you want the 3 parts to be treated as three separate entities and you def don't want that.

There is a pretty neat script that does sitemaps on auditmypc.com so i wud start there.

What Im not quite sure about is, my site will be changly several times a week. suerly if I generate a sitemap using the suggested tool it will be out of date/wrong as soon as I create a new blog post or add a page?