Daryler,
Secondary Extensions by default work for any type of call, Internal or External. I am not sure there really is a simple solution to the problem you are having, but the only reason I can think of that you wouldn't want to use a secondary extension is this.
If a call comes to the phone that is being monitored and you answer it with your cell phone lets say. Also, at the same time you answer with your cell phone both users try to answer the call as well. What they will get is each other as one rings the phone and the other answers it. The secondary extension feature is more of an executive secretary feature and is mostly intended for one person to have a secondary extension key of another person, but you can use it as you intend too.
Here is the information for how Secondary Extension is programmed for an extension.
Secondary Extension Key
Two fields are programmed for this type of button:
Selection: Select the primary phone that will be associated with this secondary extension button. The primary phone must be on the same node as the phone being programmed.
Ring When __ Calls At Extension: This field allows the phone to receive a burst of ringing when "n" number of calls is present at a primary phone, and at least one call is ringing or camped on. (That is, a setting of 2 will cause the phone to ring when the primary phone receives a third call, while two other calls are ringing.) The ring burst will repeat periodically as long as "n" number of calls is present at a primary phone. (This period is determined by the DKTS Secondary Extension Key Alerting Tone timer.) If this field is set to 0, the secondary phone will never receive the burst of ringing.
The only simple solution I can thing of off the top of my head it to put all [3] phones in a Hunt Group and then give out that number as your extension number. Alternately, you could just change your extension number and give the Hunt Group your old extension number. Also, make sure that everyone who has a DSS/BLF of your phone is change to the Hunt Group. This should work with your Dynamic Extension since it doesn't care how the call gets to the phone once it is turned on. The only thing to remember is to create a Call Routing Annoucement [CRA] that has no recordings set for day or night that will time out to your mailbox. Put that CRA in the recall of the Hunt Group and then set the Recall timer to 16 seconds, approximately 4 rings.
Hope that gives you some ideas and direction to go on