In the UK, current food labelling uses the "Guideline Daily Amount" declaration system for nutrients. This system is different to that used in other countries and has it's drawbacks, but it's simplicity allows it to convey the essential information to the public.
The problem with "sugar" is that it is not possible to declare a true daily recommended upper limit of consumption. This is because the word "sugar" includes conventional table sugar (sucrose), fructose, lactose and many others. In addition, sugar is "hidden" in many foods such as fruit, and it's simply not possible to calculate intake from sources consistently and accurately. Food processing methods also make it difficult. However, the most important reason is that because the body actually manufactures all the simple sugars we need, we really need no sugar intake at all, which makes giving such a figure pointless
Having said all that, currently the Guideline Daily Amount total for all sugars is 90 grams.
http://www.tesco.com/gda/gda.asp
The UK Food and drink Federation "What's Inside?" campaign comes up with the same figure.
http://www.whatsinsideguide.com/The5keynutrien ts.aspx
In view of what I said above, this 90 grams can therefore be regarded as the maximum guideline daily amount per day.
So you're right - drinking three cans will take you over the sugar GDA.