I want to connect a new 110 volt pagoda light between two others in series. What do I need to do this?

I have two pagoda lights along a driveway. I want to install a thiord between the existing pair. How do I do this?

If they really are in "series" as you say, if you unscrew the bulb in one, the other one will go out. I think you mean they are on the same circuit (but in parallel). Next, how is the two lights connected. (Underground wire or overhead wire)?
Basically you need to connect the new light in between the two existing ones. This means you need to break the wire in between. Now you begin to have a problem. The two wires will be too short. You will need to splice them and a new wire to the new fixture.
Since this is outdoors (in the weather) you will need a weatherproof box and cover, sealant and a good knowledge of what you are doing. You will need to be sure the wiring is for outdoors. (Generally not the rubber extension cord type).
Best, do all the mechanical work IE; set the light, dig up the existing wire so you can see it and call an electrician.

you can buy squeeze on wire taps that works really well on a project such as this go to a home center talk to a knowledgeable clerk they'll steer you in the right direction. good luck

well, as best that I can tell from limited info on this question, I would have to say, first you need to wire them in a parallel circuit, not in a series, In series circuits voltage drops as it passes though each device, Parallel circuits share the electrical load with all installed devices, Series would be ok if your light fixtures dont have any internal transformers, just a bulb, what brand and model are you using