A couple of things I'd point out.
1) Charter boats are used very hard, often by people with very limited experience and skill at sailing a boat.
2) Charter sailboats often have a different cabin and interior layout than most private owners would prefer to get. Many charter boats will have a three-cabin layout, with two smaller aft cabins, rather than a larger, master cabin in the aft section. You should visit the boat with each layout to see if this is acceptable to you or not.
You don't say how much you will get each month. Some of the charter-to-purchase plans are better than others. Some guarantee a certain level of income during the charter period, others are dependent on how much your boat is used. The location of your boat will have some effect on this, as less popular locations won't be paid as much as more popular locations.
You also don't say how much the buyout for the boat, after the three years of chartering it is going to be. Given, that you will have a three-year old boat, with a lot more use than a privately owned boat, with a fair amount of gear in need of replacement or upgrading, the price for the buyout should be fairly heavily discounted.
Some of the sailing magazines have done comparisons of the charter-to-own programs in the last year or so. Might be worth while to look them up.
Also, my last question for you is what kind of sailing do you intend to do with the boat in three years time. If your goal is to have a decent blue-water passagemaker and cross oceans or circumnavigate, the boats in the charter-to-own programs are probably not the best fit for what you want to do. Most of the boats in the charter business are not much better than floating condo coastal cruisers, with no serious bluewater capabilities.
If the type of sailing you plan to do is go from marina to marina along a coast
line, then the charter type boats may be very good for you.