For the latest Cisco 2700 series AP, if you power it up with standard 802.3af PoE switch, you can only achieve 3x3.
However if you power it up with 802.3at PoE switch, or using power cube, you can achieve the maximum performance 3x4.
How about power injectors?
The answer is yes for power injector 4 (AIR-PWRINJ4=), you will get 3x4 with power injector 4. However you cannot get 3x4 with power injector 5 (AIR-PWRINJ5=), because power injector 5 is designed to provide 802.3af power only, you will only get 3x3 in this case