Before starting you must read What is Root in Android .
This method will work on Samsung Tab 3 SM-T210 and SM-T210R . You should know the difference between 3G and WIFI based Samsung tablets.
Preparation for Rooting .
Download Odin 3.09 .
Download T210.TWRP.2.8.0.0.tar.md5 (TEAMWIN RECOVERY PROJECT)
Download Update SuperSU .
Copy UPDATE SUPERSU to the root directory of your tablet internal or external memory.
How to Root Samsung Tab let 3 SM-T210.
Turn on your Samsung tablet into Download Mode by pressing volume down + home + power button. Connect with computer .open Odin and let it select com .
When you see "Added " on Odin , click on AP|PDA and select T210.TWRP.2.8.0.0.tar.md5 File .
Click Start to upload TWRP .
After completing installation , a PASS!/RESET! message will appear and galaxy tabwill be rebooted.
You have successfully install TWRP Recovery on your tablet, but you are not done yet.
Turn off tablet and turn on into Recovery mode by pressing Volume UP + Home + Power . You will be redirected to TWRP Recovery .
Click on Install and select UPDATE SUPERSU from tablet internal or external memory .
Slide confirmation slider button to flash SUPERSU on your galaxy tablet .
When done reboot your tablet by click Reboot --System .
Tablet will be reboot and SUPERSU icon will appear .
Note :- The above mentioned tutorial and procedure was tested by me. Found no harm or error during process. However the procedure was 100% safe and secure but "backup your data first", is always recommended.