I just changed my transmission fluid and filter at 80,000 miles. I would say that unless you really do a lot of towing, you can stick to the 10 year 150,000 mile service interval. The old oil wasn't that bad, not as clear as new and smelled worse but nothing like old engine oil and new oil. I was interested to see if there was any debris in the filter. The filter is built into the pan, a one piece plastic finned sump with two big magnets. There was a slight amount of fillings around the magnets but nothing like what I found on the differential magnets.
Changing the fluid is the hardest fluid change to do on an LR3 but not too difficult. The tranny cross member needs to be removed. To do that you need to support the tranny with a jack. After the cross member is removed, the tranny needs to be lifted about another inch or more so that the pick up tube on the filter/sump/pan can be pulled down out of the tranny. To do this safely, the whole frame should be supported by four jack stands. You don't want the air suspension to adjust while the engine and transmission are held up by a jack. Unless you have a well equipped shop, this might be a job left to a professional. There is nothing special about the transmission, any shop that works on European cars should know the unit ( a 6HP26 I think).
It takes 6 quarts of the LR specific fluid at about $28.00 each.