Assuming they are on separate servers, then yes you would have to install 3 forwarders and set them up to monitor the directory that each application logs to.
The apps are used to make your data neater and easier to search by making the fields CIM compliant. Most of the add-ons have pages within the docs that tells you where the add-ons need to be installed. Below is the MS IIS add-on page:
https://docs.splunk.com/Documentation/AddOns/released/MSIIS/Install
Hope that helped!
