The differences between the GRC-3 and the GRC-7 are the frequency of the Receiver-Transmitter and the Radio Receiver.
For the Amor units the RT-66 with the R-108,
for the Infantry the RT-68 with the R-110.
1st Battlegroup, 18th Inf. use the GRC-7, I would say.
The dimensions (mounting,PP-112, AM-65, RT-70 etc.) are the same, also the antennas.
Frank