There are three ways in which we can mix our conditionals.
1) To talk about something that didn't happen in the past but which would have a hypothetical result in the present.
2) To talk about something that hasn't happened in the present but has a hypothetical result in the past.
3) To talk about something which won't happen in the future but has a hypothetical result in the past.
Type 1 example:
If I had taken physics at school, I'd have a job as a lecturer now.
~ if clause is 3rd conditional ~ result clause is 2nd conditional
To talk about a hypothetical situation in the past with a hypothetical result in the present.
Type 2 example:
If I were funny, I'd have become a comedian.
~ if clause is 2nd conditional ~ result is 3rd conditional
To talk about a hypothetical situation in the present with a hypothetical result from the past.
Type 3 example:
If I weren't visiting my family next week, I could have gone on holiday to Barbados.
~ if clause is 2nd conditional ~ result clause is 3rd conditional
To talk about a hypothetical situation in the future with a hypothetical result from the past.