Depending on the discount you get when you buy (how many at once), and the lease price you're paying, it takes roughly 7-9 years for the monthly payments on a lease to equal those of a purchase.
So if you only plan on keeping the plane a short while, there's not a particular reason to buy it. Of course, even if you don't need it, you can always turn around and lease it yourself -- but if it's not a plane anyone wants you don't get anything. Or maybe can even sell it yourself, in which case the break-even point seems to be in the 3-4 year range -- i.e. you can sell a plane after 4 years, and the difference between what you bought it for new and what you sold it for used, is less money than it would have cost you to lease it for that same 3-4 years.
There's also cost of money to figure in as well. If spending all your money on buying 3 planes outright prevents you from leasing 15 planes that could be used to significantly increase your revenues and market penetration the financial security of the purchase may not be worthwhile. But that's up to you to decide.