So you just replaced one CAS platform with 3 and in the process quadrupled the crew required, tripled the personnel being shot at and at the very least tripled the operating cost. Unless you are assuming there is only one of the three on station at a time and therefor fielding a third of the capability?
Currently the US airforce is considerinfg adding more aircraft in the CAS role and are looking at aircraft like the A-29 Super Tucano or OV-10 Broncos to do it.
Source
AH-64 & Drones are existing platforms.