That is very easy. Determine rate of change in the size of the database during the past year. Declare an assumption that the rate of growth or decline will continue for the foreseeable future. Then multiply the annual rate of change by three! -- that is the database size after three years!!
See you can follow this method if it suits you. If you or anybody has been maintaining a capacity planning for the db, in sense what is the size of the db at the end of each month, then things become very easy. You can calculate the actual GB increase month on month or the % of growth every month. Or also at the data level you can calculate how much data is inserted into the db every month. Then you can extrapolate this for the coming 3 years.
Also take into consideration that you may want to add 25-30% additional capacity on top of this(10% for new tables/columns being added,10% for quarter/year end data spikes ,10% for contingency) for each year and then reach the final estimated value.