that may be overkill and also may not work as efficiently as you might think if you're not able to maintain a good constant pressure for each membrane. I would consider 2 at most if you insist on it. Should be shooting for that 70-80psi constant pressure.I have an un-setup system with 3 (yes 3) membranes
Yes! Go with 3 DI canisters. You will have cleaner TDS for much longer. Your first two will get smoked fairly quickly, buy that 3rd one last significantly longer. I'm using cation and anion in my first two and mixed bed in the 3rd.and 3 clear cups, trying to decide if i should get the expansion 2 more DI canister cups for $30
I would also consider adding an additional method to flush the freshly made RO before your first DI canister. Just to eliminate the inevitable high TDS that will get through during startup and even after an initial system flush. Doesn't take long but it will extend the life of your resin.