Work with Salesforce Prefixes.

SalesforcePrefixUtil.apex

//Find an entity's prefix
String prefix = MyObjectType.SObjectType.getDescribe().getKeyPrefix();
System.debug(prefix);

class PrefixFinder{
	public String findObjectName(String recordIdOrPrefix){
		String objectName = '';
		try{
			String providedPrefix = String.valueOf(recordIdOrPrefix).substring(0,3);
			Map<String, Schema.SObjectType> allObjectsTypes =  Schema.getGlobalDescribe(); 
			for(Schema.SObjectType stype : allObjectsTypes.values()){
					Schema.DescribeSObjectResult describeResult = stype.getDescribe();
					String prefix = describeResult.getKeyPrefix();
					if(prefix != null && prefix.equals(providedPrefix)){
							objectName = describeResult.getName();
							break;
					}
			}
		}catch(Exception e){
			System.debug(e);
		}
		return objectName;
	}
}
//Given the prefix, find the object.
String objectName = new PrefixFinder().findObjectName('001');