I'm no expert on the matter but I'd imagine that a lot will depend on how it has been stored for the last 3 years. Has it been exposed to the elements/UV or has it been stored under cover or inside a building? Have the tubes been kept inflated or not? Is the seller/manufacturer offering the full warranty from date of sale, or date of manufacturer (if the standard is 5 years then you could be accepting just a two year warranty)? If an engine is fitted, is it new or also 3 years old, and if the latter is the warranty similarly affected?
If it was me, I'd be looking at what a three year old model would sell for on the secondhand market, and deciding whether any premium thereover was reasonable for its relative condition and lack of use.