Custom recovery for any android device including tablet or phone allows one to flash custom firmwares, ROMs and all sorts of hacks. Same are now available for the T110, T210, T211, T217T versions of the Samsung Galaxy Tab 3 7.0 android tablet. Download links are given at the bottom of this post. 
You can flash the Galaxy Tab 3 7.0 TWRP CWM recoveries using ODIN tool. This is a desktop tool that lets you flash a custom recovery. Only after that, you will be able to flash your device.
Download links: