Where can I study three languages at uni?

Lots of unis offer three languages; not as many offer Russian though. You'll probably want to start by seeing which unis offer Russian and then see if you can combine it with two other languages. Bear in mind also it may be possible to take the third language as optional modules in a joint honours degree between two of them. So that might be worth looking into (I think this might be possible at Leeds for example).

Also look at Scottish unis - in the Scottish honours system, in the pre-honours years you can usually combine several subjects (including potentially 3 languages) before specialising in one or two for your honours years. You could for example then do all three language in pre-honours years at e.g. Glasgow, Edinburgh, Aberdeen, etc (assuming they offer them all - I don't know that!) and then "drop" one before honours years (you might be able to keep it up in some capacity through optional modules, again, but it won't be part of your degree title). Particularly if you are studying one or more at post-A-level, it might make sense to keep any languages started from scratch through the whole degree, then whichever one you're strongest in alongside that