Get all SObject's prefixes in the Salesforce ORG. Better to be used like a SysAdmin.

SObject Prefixes.cls

Map<String, Schema.SObjectType> gd = Schema.getGlobalDescribe();
Map<String,String> objNameToPrefix = new Map<String,String>();
for ( Schema.SObjectType ot : gd.values() ) {
	Schema.DescribeSObjectResult dor = ot.getDescribe();
	objNameToPrefix.put( dor.getName(), dor.getKeyPrefix() );
}
System.debug( '--->>> ' + objNameToPrefix );