Welcome to the forum.
If you are 100% certain that you have no Orphans (even a 1 Frame Orphan, which would be almost impossible to see), then the only thing that I can imagine is that the WAB (Work Area Bar), has become set out beyond your video, to ~ 00;03;00;00 beyond your last Frame. If one is Exporting/Sharing the entire Timeline, that should not be an issue, but if they are doing the Export/Share for the Work Area, I could see where it would possibly cause a problem. Check out the WAB too.
There is an obscure function, which sets an In and Out Point on the Timeline. This is NOT the In & Out Points on a Clip, but on the Timeline. This function is use for 4-point editing in Premiere Pro, but really has no use in PrE. It can accidentally be set, by having the CTI in places on the Timeline, and then hitting the I ("eye" key), for In Point, or the O ("oh" key), for the Out Point. Those Timeline In & Out Points are not easy to see. Basically, when they are set, the Timeline Ruler bar has a slightly darker area displayed. It is only obvious, when viewed next to the regular Timeline Ruler bar. If you had a longer Timeline, before all of your Clips were Trimmed, and accidentally hit the O key, you could have set a Timeline Out Point, that would be hanging out beyond your last Frame. Check carefuly for any "discoloration" in the Timeline Ruler bar, out around where you are getting your End.
Other than the Orphans, or the Timeline Out Point, I cannot imagine what could be happening to cause your Timeline to Export/Share with 3 mins. of black, after your last Frame.
Good luck, and please let us know what you find out there.
Also, in the Program Monitor, look at the TimeCode (lower-left in the Program Monitor Panel), and see what the total Duration is there. How does that compare with where your intended last Frame is located?
A quick and dirty workaround would be to set the WAB running from the start to where you actually want the end. Then, during the Share, tick the checkbox to share WAB only.
Insanity is hereditary, you get it from your children
Thanks I can't see any disoocloration and the timecode looks ok in the program monitor. I'll try Neale's fix and let you kow if I fail again!
I was just looking at possibilities. Those Timeline In & Out Points are not common, and usually show up here in threads along the line of "what is this darker gray box... ?"
Where was your WAB set?
What did the TimeCode counter show for the full Duration of your Timeline?