If the Galaxy Tab 3 7" can read a USB Flash Drive, What is needed and where can I get whatever is needed. If there is a specific app please let me know.
Tab 3 should support OTG with its micro USB connection. You just need a micro usb to usb adapter.
File format needs to be exFAT for Android to see it, some flash drives are formatted NTFS because that is what Windows uses.
The tablet should have a file browser on it already, either called files or my files. Use that app to view whats on the flash drive and copy/ transfer files.