The current of the primary side in your case shall be 3000*1000/(1.732*11000) = 157.45 amps
so,say that the rating of the primary protective device (fuse for example) shall be 200amps for example (you have to check the characteristic curve of the protective device to avoid tripping during the starting in-rush current, moreover the 200Amps fuse must be rated at least 11KV and have correct short circuit interupting breaking capacity accoding to the maximum calculated short circuit level of your installation).
The first thing to do is to select the size of the cable is to choose a cable with a current carrying capability (after using appropriate derating factors) greater than the rating of the primary protective device (200amps) and can withstand the primary voltage (11KV).
Then, you have to calculate the voltage drop of the cable and make sure it does not exceed that determined by your local standards.the voltage drop of the MV side is usually very small and can be neglected.
After that you must calculate the maximum short circuit current which the cable might be subjected to and select a cable which can withstand the short circuit current until the protective device clears the fault (usually 1 sec).