can someone pls explain the difference and usage of these words? I never saw an explanation on how to use them correctly. Right now, all 3 are just "(some time) ago, previous, before ..." for me.
Just to clarify「先」really is only used for future reference and not for past reference. There are examples that will take on that meaning such as,
The previous emperor was Emperor Hirohito
But, just to be clear, it may be worth learning for understanding ,but is rarely used in every day life. This is the way it is used 99% of the time.
He left first
It also has the meaning of "tip" as Brandon states
「さっき」Refers to immediate past tense situations,
He just left
「前」Can refer to a time further in the past or immediate past.
He just left a few moments ago
He went to Japan a year ago
And you can use「前」as the "front",
Please sit at the front of the room
先 can talk about the past or future and has a great many other meanings related to its fundamental meaning of something a bit like "fore", "head", or "tip".
さっき only talks about something in the (very) near past, and is very close to さきほど in meaning.
前 can talk about any time period in the past -- near or far.
Not sure if that's enough detail, but looking at examples is the best way to learn the detailed differences (先 is the most difficult to use correctly / naturally with time IMO).