There are thousands of models of universal remotes. There's more than one scheme by which universal remotes call up sets of IR codes that are all stuffed into a memory chip inside the remote.

The instructions for the remote you bought should list the three-digit codes FOR THAT REMOTE that will allow it to be changed so it controls the DB840. And that's what's happening -- the commands are (maybe) stored in your universal remote, and entering the proper three digit code will make that remote send out codes that work with the DB840.

But sometimes, not all model numbers are given. In that case, the instructions for the remote should detail a process of trying one code after another until one works.

I don't know why you're looking online for the three digit number. You don't tell us where you're looking; if anywhere, it would be the site for the manufacturer of your universal remote.

Follow all possible instructions in the universal remote's manual. Then, if nothing works and you tell us the make and model of the remote, maybe we can help. And drop me a PM so I'll come looking here to respond to you.

I'm quite sure the Sony site says "The remote control is not programmable and remote codes are not required" because they're talking about a SONY remote, not any one of a half dozen brands of universal remotes, which do need codes (sometimes three digits, sometimes more)