I want to play Mass Effect 1 using the character I have made in Mass Effect 3. Anyone knows how to? Maybe like by copying files or something?
Each of the Mass Effect games made changes to the character files which are irreversible without hacking/editing the files yourself, or finding tools to do so. However I'm unable to find any such tools.
As others have said, you can't import an ME3 character to ME1.
If you're primarily concerned with making the two characters look the same, take note of the letter and number string in the corner of the character design the screen
It represents the facial characteristics of your character.
If you write it down, you can use it to manually recreate the character in ME1.
(This is how I managed to maintain my ME2 Shepard in ME3 before they fixed the importer bug.)