This is a very common requirement in workflows and it is handled easily in blackpearl.  Check out the following technical article found here on underground: http://www.k2underground.com/files/folders/technical_product_documents/entry20948.aspx

There are several ways to do this, but here is the approach I generally use. In the sub process, I have two parallel paths from the start.  One path is the normal workflow.  The other path leads to an activity that contains a SmartObject Event and two Server Code Events.  The SmartObject event writes the serial number and a ...

OK,so it looks like you really do need 3 activities.  The reason your IPC event isn't finishing is because the other client events are still waiting for user action.  Here's one way you could accomplish this. Make all of your client events join at one activity somewhere down the line.  This activity will use the GotoActvity method ...

Is it the same client event for each of the users?  If so, you might want to consider doing this in one client event with one slot, but have three destinations.  That way, the first one who finishes completes the activity.