The least expensive option is to use your single copy of FileMaker to host the file using Instant Web Publishing. Then the other two users can use their web browser to access the file. There are design limitations in due to useing browsers instead of copies of FileMaker, so you will need to read up on instant web publishing and plan on an extra round of testing to make sure your layouts work with the web browsers used by your other two users. There's a PDF that comes with FileMaker that has more info on IWP. Pull down the help menu and select the documents sub menu to find and open this PDF to learn more.
You'll also need to set up port forwarding on your internet router so that your remote users can find and connect to it in order to access your web published database.
For security, you can use manage security to issue each user an account and password to use for controlled access to your database.
You may also want to consider using a FileMaker hosting provider, such as ourselves. If you want more information, please contact me privately. Thanks!