trigger Sample on Record_Type__c(before insert){
    String[] names = new String[]{'First Record','Second Record','Third Record'};
    Map<Id, RecordType> regions = new Map<Id,RecordType>([SELECT Name from RecordType WHERE SobjectType = 'Record_Type__c' AND Name IN :names]);
    AggregateResult res = [SELECT MAX(Record_Number__c) max1, MAX(Record_Number1__c) max2,MAX(Record_Number2__c) max3 FROM Record_Type__c  WHERE RecordType.Name IN :names group by RecordTypeId];

    Integer max1 = Integer.valueOf(((String)res.get('max1')).right(3) );
    Integer max2 = Integer.valueOf(((String)res.get('max2')).right(3) );
    Integer max3 = Integer.valueOf(((String)res.get('max3')).right(3) );

    for(Record_Type__c theRT: trigger.new){
       

            if( regions.get(theRT.RecordTypeId).Name == 'First Record'){   //region corresponds to 'US'
                String num1 = String.valueOf( (max1 != null) ? ++max1 : 1 );//will use the incremented value of max unless max was null (else use 1 instead)
                for(Integer i=0; i< (3 - num1.length() ); i++){ //should pad with leading zeros to ensure format of {000}
                    num1 = '0' + num1;
                }
                theRT.Record_Number__c= 'A - ' + num1;
            }
            else if( regions.get(theRT.RecordTypeId).Name == 'Second Record'){    //region corresponds to 'ROW'
                String num2 = String.valueOf( (max2 != null) ? ++max2 : 1 );
                for(Integer i=0; i< (3 - num2.length() ); i++){ //should pad with leading zeros to ensure format of {000}
                    num2 = '0' + num2;
                }
                theRT.Record_Number1__c= 'S - ' + num2;
            }
            else if( regions.get(theRT.RecordTypeId).Name == 'Third Record'){    //region corresponds to 'ROW'
                String num3 = String.valueOf( (max3 != null) ? ++max3 : 1 );
                for(Integer i=0; i< (3 - num3.length() ); i++){ //should pad with leading zeros to ensure format of {000}
                    num3 = '0' + num3;
                }
                theRT.Record_Number2__c= 'T - ' + num3;
            }
       
    }
}
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when i create an record it shows an error====
Error: Invalid Data.
Review all error messages below to correct your data.
Apex trigger Sample caused an unexpected exception, contact your administrator: Sample: execution of BeforeInsert caused by: System.QueryException: List has more than 1 row for assignment to SObject: Trigger.Sample: line 4, column 1