The AWR1642 supports Advanced Frame Configuration that can be for your purpose of sending 3 different frames in a sequence.
You will need to configure different SUB-FRAME using the advanced frame configuration sub-block.
Please refer to Section "5.4.6 Sub block 0x0105 – AR_ADVANCED_FRAME_CONF_SB" in the "AWR1xx_Radar_Interface_Control.pdf" which is available in the MMWAVE-DFP for the message description .
