General speaking, it should be less efforts on RF consideration when make the mating PCB layout for AT3.To layout the AT3 with MPS430 in serial interface, just needs following the basic PCB design rules, we do not have the certain number to specify the traces lengths of the serial lines. But it should be as short as possible. The DC supply voltage should be de-coupled as close as possible to the Vcc pins with high performance RF capacitors (recommend SMD 0.1uF). It is preferable to mount a large surface mount capacitor (e.g. 4.7uF tantalum) in parallel with the smaller value capacitors. To protect SUSPEND from transients,
a 0.1uF capacitor decouples to SUSPEND pin of AT3 is recommended.
All device grounds, Vcc/VDD connections and VDD bypass capacitors must be connected as close as possible to the AT3 module. Long power supply lines on the PCB should be avoided. For a PCB with a topside RF ground plane, the GND pins should be connected directly to the ground plane. For the PCB with a bottom ground plane, the best technique is to have via holes as close as possible to the AT3 GND pads.It is best to position the AT3 module so that the copper keep out area is on the edge or over hanging the edge of the mating PCB. If the copper keep out layer is completely within the boundaries of the PCB, the ground plane should not fully surround the copper keepout. If this is not possible, the copper keepout should be made much larger to prevent RF degradation. The ground plane is recommended to extend to the full size of the mating PCB, with the exception of the copper keepout.
Full swing digital data or control signals should not be routed to the crystal or the power supply lines.
Unclear what is the purpose to put the regulator on the AT3. If it is intended to control the radio RF power, you can use the ANT power control message to adjust the radio outputs from 0db to -20dbm. The typical work voltage for AT3 is 3V, the lower DC supply will cause the current consumption goes higher. For dynamic voltage adjustment, you can consider use a DAC pin of MPS430 to control the FB pin of the TPS780 making the adjustment.
You can check the datasheet of TPS780 for more detail.