Fluorescent bulb lighting fixture has 3 two-bulb sets One set does not light when you turn on the switch yet when you twist either bulb in its socket the pair will light so do I need to replace bulb?

Answer 1

Are we talking about long fluorescent "tubes"?

The socket may be bad, or the tube is not seated correctly in it's socket.

Some fixtures use a socket that the tubes "twist" into.

Twist the tube slightly in either direction until it seems "seated".