Here are the corrections for each of the sentences:

1. I was going (A) to work **by** (B) motorbike, when I was seeing (C) a (D) car accident last month.
- Correction: **B** (should be "by" instead of "buy")

2. The house in (A) the city is (B) too expensive for (C) **us** (D) to buy.
- Correction: **C** (should be "us" instead of "we")

3. Jane did (A) a lot (B) of mistakes (C) in her composition because of (D) her **fascination**.
- Correction: **D** (the word "fasiness" is incorrect; it should be "fascination" or another appropriate term)

So, the letters that need correction are:
1. B
2. C
3. D