Thanks to XDA dev DarkFrenzy,for porting Cm12.1 rom for Galaxy Tab 3 7.0.There are some major bugs and some minor bugs in it that are Major:Camera not working and Screen flickering.Minor:You will see the screen rotates to landscape mode while locking the device,if you try to seek while watching videos using HW in MX player it will
take sometimes to come to the seek place. (so use HW+ or SW Decoder to
fix this bug.For latest Bug report i suggest you to visit XDA source Thread Here
#How to install Unofficial CM12.1 on Galaxy Tab 3 7.0
1.Download official CM12.1 Rom for Galaxy Tab 3 7.0 from below links.
2.Now place the Cyanogenmod 12.1 rom file in sd card.
3.Now reboot to TWRP Recovery mode
4.Then flash CM12.1 android lollipop rom from TWRP recovery in it.
5.Now Clear cache and Clear Data.
6.Reboot your device.Done! you just flash cyanogenmod 12.1 rom in your Galaxy Tab 3 7.0.Enjoy Android lollipop 5.1.1.
Note there is no need to install gapps it is already in the rom.
1.Download Cyanogenmod 12.1 rom for Galaxy Tab 3 7.0
( download the latest from here ).