3. If we had taken a taxi, we wouldn't have been late.

Solution:

This is the third conditional. To omit 'if' and use inversion, we start with 'Had'.

1. Original: If we had taken a taxi, we wouldn't have been late.
2. Inverted: Had we taken a taxi, we wouldn't have been late.

Answer: Had we taken a taxi, we wouldn't have been late.