Forms :: Concatenate Or Copy Value On Enter Or WVI

I have three database fields on my form , field_1 , field_2 and field_3 , user will enter the data in field_1 , and sometime he may not enter in field_2 ,

I want to bring the value of field_1 as it is into field_3 if he does not enter anything in field_2, if he enters the data in field_2 , then field_1 will get concatenated with field_2 and gets copied into field_3.Is there a way to do this automatically at form level using a trigger when-validate-item.In short this should be done after user presses the enter key on field_2.

create table ot_Sale_code_head ( sale_code varchar2(12),code1 varchar2(6),code2 varchar(6),code3 varchar2(12));
insert into ot_Sale_code_head ('A','01.01',NULL,NULL);
--inserted record in form will be 'A','01.01',NULL,'01.01'
insert into ot_Sale_code_head ('B','01.01','PS-P',NULL);
--inserted record in form will be 'B','01.01','PS-P','01.01.PS-P'