Please make sure you allow sufficient time for the data to transfer from the PC to the Droid 3!
Since, you have a Droid 3, you have two possible choices as to where you store pics, music, videos, etc. These are
1. Internal Phone Storage aka "media area" (some apps call this SD Card and some file managers call this 'sdcard')
2. External SD Card; optional user-installed micro-SD card (some apps do not recognize this and some file managers show this as 'sdcard-ext')
The Gallery app and 'Music' app can read for both the Internal Phone Storage and the External SD Card. So far every app I use for pics, videos, and music can locate and read the data from both locations.
Since my External SD Card on the Droid 3 was moved from my OG Droid, I continue to use the same file structure as was on my OG Droid - Music is in 'My Music", pics in "My Pictures", and Videos in My videos.
I move all pics and videos from internal phone storage to the External SD Card.
Pictures and videos taken with the camera are found in the internal phone storage folder DCIM.
Pictures and videos sent via text messaging the internal phone storage folder Messaging.
If you are having problems transferring files from the PC to your phone, I suggest you try the following;
1. Plug your USB data/charging cable in to the PC, then plug it into your Droid 3, Pull down the Notification/Status and select the USB Connection. Make sure the 'USB Mass Storage" is selected.
Now you should be able to drag and drop from the PC to either the Internal Phone Storage (shows as device MOT) or the External SD Card (shows as device Removable Storage).
2. Make sure you are using the correct data/charging cable for the Droid 3. Some third-party cables and cables for other phones do not work. I know that the USB data/charging cable for the LG ENV3 does not work for data with the Droid 3. I also had a couple of third-party cables that did not work. (I tossed all my cables that do not work with the Droid 3).
Good Luck,
Bob