create webform that allows user to create either 1, 2 or 3 activities depending on user selection
You can accomplish this by:


For each activity, make the Activity Type field "user select".
Edit those 3 Activity Type fields on the webform. For each one:


Deselect "list box"
Deselect all activity types except the one you want.
Rename that option to something like "Create activity"

Add a conditional rule on the form showing/hiding other activity fields depending on the Activity Type field being checked.


This will present a checkbox on the form allowing users to select whether or not they will create each activity.

Note: This works in the current version of webform_civicrm but logs an error saying "could not create activity." I've fixed that superfluous error report in -dev (as well as fixing a bug where it could not save activity details) so you may want to upgrade to the -dev version until the next release.