pandas: top-N per group and leakage-safe rolling features

A DataFrame df has columns user_id, ts (datetime) and amount. Write code to produce:

1. the 3 largest transactions per user,
2. a feature avg_7d: each user's mean amount over the previous 7 days, excluding the current row,
3. a feature days_since_prev: days since that user's previous transaction.

Explain why (2) must exclude the current row.