How to get key prefix of an Object in Salesforce.

We know there are two types of record-id are present in salesforce.

• 15 digit -- Case Sensitive
• 18 digit -- Case Insensitive

Only 3 digit of ids represent object type .

for example 

• Account-- 001
• Contact-- 003
• Opportunities --006
• Lead  --- 00Q

How will get these prefix dynamically, I mean by using apex code ?

Schema.DescribeSObjectResult r = CustomObject__c.sObjectType.getDescribe();
String keyPrefix = r.getKeyPrefix();
System.debug('Printing --'+keyPrefix );
It will print prefix of Custom Object id.