There's no question in my mind: if you do any type of database development, use FileMaker Pro Advanced. And from one of the product info pages:
FileMaker Pro supports up to five simultaneous web users. If you need to support and share databases with larger groups of people over the web, FileMaker Server 10 Advanced supports up to 20x as many web users as FileMaker Pro.
PS: Use Version 10 - there's so many advancements, version 8 doesn't compare.
Thank you for your post.
If only three people are going to access the file, then you don't need FileMaker Server.
FileMaker Pro does allow access to your database file through Instant Web Publishing. You may want to try this locally first to see if it satisfies your needs.
It is true that there are a lot more features available in FileMaker Pro 10, but from your brief description, I can't make a judgement whether you need it. That is why I suggest you try what you have first. Specific questions will emerge from your testing.