Question

1. I saw tom three days ago.
2. I saw tom three days before.
(Are both OK?)

3. I arrived there after three days.
4. I arrived there three days after.
5. I arrived there three days later.
(Which ones are grammatical?)

6. I will meet Tom after three days.
7. I will meet Tom in three days.
8. I will meet Tom three days later.
9. I will meet Tom three days after.
(Which ones are grammatical and commonly used? Thank you.)

Answer 1

1. I saw Tom three days ago.
2. I saw Tom three days before.
(Are both OK?)

3. I arrived there after three days.
4. I arrived there three days after.
5. I arrived there three days later.
(Which ones are grammatical?)

6. I will meet Tom after three days.
7. I will meet Tom in three days.
8. I will meet Tom three days later.
9. I will meet Tom three days after.
(Which ones are grammatical and commonly used? Thank you.)

Answer 2

1 is fine; 2 would be fine if you complete the thought -- before what?

5 is the best; 3 is OK; 4 (again) would be fine if you complete the thought -- after what?

Each could be OK, depending on what you mean and if you complete the thought (9).
~ 6 and 7 seem to be referring to the future: Three days from now, I will meet Tom.
~ 8 is OK depending on what the sentence before says, so that the question would be answered -- later than what?
~ 9 needs to be completed -- after what?