Put the $$ you were going to spend on the 3rd light fixture into a
pressurized C02 setup...your going to need it with that much light
DIY won't cut it for a 75... no matter how many2 liters you set up..it will be far to inconsistent for that size tank with those lights.
And read up on EI dosing, it's a bunch cheaper and more flexible
than most other methods.
Of course, these are JMHO...YMMV... yadda yadda yadda....
Larry