If the feed is at the switch then you can just daisy chain the cable between lights 1 and 2 and then 2 and 3. How to add one or more additional lights so that all are operated from the same switch.

3 Way Switch Wiring Electrical 101
How to wire 3 lights to one switch diagram uk. Wiring for outside fixture wiring. Light two the same as light one. If you are using. This diagram illustrates wiring for one switch to control 2 or more lights. You connect the black wire from the power source to one of these brass terminals the line terminal and the black wire going to the light fixture to the other which is the load terminal. Outside lights may be controlled by motion detectors and photo cells switched from one or more locations or may be controlled by a simple home automation system.
The terminals are brass. The following will assist you with your electrical question. Adding an extra cable to the existing ceiling rose. A switch interrupts the hot leg of the circuit so it has terminals only for black wires and ground. This diagram shows the additional cable connected to an existing. Running 3 lights off one switch re.
Wiring a 3 way light switch. The source is at sw1 and 2 wire cable runs from there to the fixtures. With a pair of 3 way switches either can make or break the connection that completes the circuit to the light. Wire to that take feed to switch and back and there will still be three sets of terminals e n and swl to then take to each rose. One outside light on two switches three inside on one switch one on out side switch. Then you just need to put a switch at the upstream end of the hot wire black or red so you can shut off the flow of electricity to all three lights.
Or providing it has access being in a barn i am assuming it has you can use a junction box at each point. In your joint box diagram you have shown 2 blue wires to the lights one sleeved brown. At light one you will have 3 cables feed in feed out and cable to your light. I have attached a schematic diagram for 3 lights to be turned on at the same time by 1 switch. To add a second light which is switched with the first one the switched line neutral and earth need to be extended to the second light. The hot and neutral terminals on each fixture are spliced with a pigtail to the circuit wires which then continue on to the next light.
Wiring a 3 way light switch is certainly more complicated than that of the more common single pole switch but you can figure it out if you follow our 3 way switch wiring diagram.