Mechanical objects are meant to be used and maintained properly. Letting any motorcycle sit for 3 years is detrimental in many ways, especially if it isn't "prepped" for lack of use beforehand, and a lot depends on where and how it is stored, too.
There are no "Pros" to doing so; gas varnish can accumulate where fuel sat within the carb passageways and jets; oil drains down to the bottom of the engine, leaving critical surfaces dry, possibly including various seals, which can deteriorate and crack; other exposed surfaces, depending on the humidity and conditions stored in, can get corroded from lack of lubrication protecting their surfaces.
Tires will become hard and will need replacing, no matter how much tread is left. Your life depends on their adherence to road surfaces, and when they harden from age they are unsafe.
These types of issues can come about, but are less severe or likely in the best of storage conditions, climate-controlled and all; most people don't have optimal conditions such as these and so you should plan on riding it, or selling it, rather than let it sit for years.