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You can use a 255/55/19 for a spare, but a 255/50/19 will be too small. I would also be wary about using the RRS wheel because the LR3 has higher weight ratings on its wheels. If you're strictly keeping it on road, it will probably be fine, but just know that the RRS sport wheels are not rated for LR3 weight. Also, I have no idea what the offset is on the RRS wheels, so I don't know if they will actually work.
If you have standard wheels and tires on your LR3, then the only sizes you can use for a spare are 255/55/19 or 255/60/18. These sizes are the same diameter and interchangeable.
I am confused; all I need to know if a RRS wheel with 255/60/19 would be OK to use as a spare (if needed) when the other three wheels on my LR3 are 255/60/18