Would the diagram on the left work?

Yes, I assume that your wiring connections are in the switch box.

Just be sure the box is big enough for 3 cables.

Would the lights be dimmer on the left hand side because the power is split into 2 sets of lights?

All the lights will be of the same brilliancy, unless you have long distances 200 feet or more difference.

Normal house, all will be the same.