As a SDR TV , all 3 of them are considered adequate .
By now everyone is probably more than sick of reading pro and cons of IPS / VA based TV , so I will leave those behind.
But HDR is the hot topic in town now , we certainly can't ignore that if you have the budget to spare . Especially when all 3 of them claimed to be HDR TVs.
The Samsung NU8000 is an unusually bright TV for a mid-range model . It can overcome glares caused by the environmental light source and still display the pictures in its most natural form. Although it utilizes an edge-lit based local dimming , it can still provide a more convincing HDR effect than the other 2 TV largely thanks to its high peak brightness , which translate into higher dynamic range and is what HDR is all about.  Great price , good performance ( for this price range ) , great input latency ,  large amount of features , it is simply the best out of the 3 .
The LG UK7500 too have an edge-lit local dimming . However the dimming performance is so terrible that it causes more damage to the picture rather than enhancing it . The LG magic remote and smart TV OS are very responsive and more intuitive to use compare to the other two .
The X8500F might be a good SDR TV but it simply can't do HDR . It has steady brightness output but due to the absence of of proper dimming system , it can't produce an accurate impression of HDR . It also has a poor black uniformity just like all the other IPS TVs . Other than that , it remains a good pick if you are mainly into watching low quality streaming shows and fast action movies due to the excellent picture processing from Sony TVs.

Assuming that you don't need the Android TV OS ,  the UK7500 does offers a few more features and upgrades over the Sony basic XF75.
For starters , the UK7500 is better than last year's UJ7500 since LG has also stopped using RGBW in their mid-range TV ,  so it is a full fledged RGB panel now .

The UK7500 is considered as LG 's mid range TV and has more HDR related features such as Local Dimming & Nano Cell ( which is LG 's term of supporting a Wider Color Gamut ) .
To sum it up  , if you don't watch HDR content at all just buy whichever that is cheaper .

If you plan to watch some HDR related content from Youtube , Netflix , Amazon Prime & Vudu , the UK7500 will server you better.
If you are a big sports fan and buying the TV mostly to watch sport stuff , the Sony TVs are usually better because of their excellent motion processing .
For everything else , the UK7500 is more appropriate for the task.
Despite of what LG says , RGBW 's goal is to save production budgets .

LG is fully aware that the RGBW panel has harmed their brands and reputation more than goods , you would expect them to after getting into a big fight with Samsung.  So this year they have stopped using RGBW in their mid range TVs.
Only the UK6100 and UK6300 are using RGBW panel this year , everything else are full RGB now .
For laughing purposes :





And of course , don't take any of them too seriously . As usual , there are a lot of exaggerated bullcrap in both of them.