"Three days ago" always means "three days before today". You can't use it with any other time except the present. Ex: "I bought a plane ticket three days ago." The equivalent for the future is "three days from now". "Three days earlier" isn't used with the present and it always needs context. Ex: "I went to a concert last Friday. I bought the ticket three days earlier." The equivalent for the future is "three days later". If you say "three days before", you have to put the context right after it. "I bought the ticket three days before the concert." The equivalent for the future is "three days after". But you CAN'T say "I bought the ticket three days earlier the concert." You CAN say "I went to a concert last Friday. I bought the ticket three days before." Even though you usually have to put the context right after "before", you can leave it out in this case since you just mentioned it.

both your examples are correct. the differences between ago, before and earlier are quite small. For example, you wouldnt really say "i did it ago i went out" but "i did it before i went out" is correct english. You can also use "before" like this "I've done it before" you could also say "i did it earlier" but "i did it earlier i went out" is wrong. ago can only really be used in examples like the one you wrote, or you can also use it in some questions like "how long ago was it that you last watched tv?"

Not much difference. 3 days ago would suggest its 3 days from this moment. 3 days earlier ( or before ) could refer to another specific time in the sentence. I saw his last saturday and 3 days earlier.