Load rating of LR3 rims is 940 kg
Look about two thirds way down re this link to a couple of files on wheels and ratings.
As per the post above, the 3 is the most heavy Land Rover there is and hence, while other LR wheels will physically fit, they will most likely be of lesser load rating. 20" rims off an LR4 would be rated OK however.
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disco3 co uk and insert a period in each of the two spaces.
If you really want differerent wheels, you might try finding some 18" LR3 factory wheels. You probably will not be very successful however as they can be like finding hens teeth. If you did find a factory set, you could probably trade them for 20" LR4 wheels and maybe pick up some change as well. If instead you decided to install the 18" wheels, the ride would improve and most likely the handling on normal roads as well. With the 18", you can then install the narrower 235 65 R18 XL tires and improve winter traction with the appropriate tyre installed.