i want to add the three fields into like eg firstname, middlename, lastname into one field as full name

yes you can do that by saving them separately first and on button action you can save them to the particular field using concat()

Hi,

just set a Text (or Rich Text) Field's value as concat(ri!first," ",ri!middle," ",ri!last) or ri!first&" "&ri!middle&" "&ri!last

did i need to create a custom record fields for that

for the Full name fileld

What exactly you want to do just show or you need to save it as well

like i have the three columns in the database table but i just one column to be displayed into the my main application as Full name

You can create a customer record field by concating the three different fields if you want to use it often and also want filtering working on it. 

which template should I choose and can you please help me with the expression 

I think you can write an expression on your own without a template and I am not sure if there exists a template for it. 

yes i got the answer