The FONA 3G adds 3G support but since it is a completely different chipset than the FONA 800/808 (Qualcomm not Mediatek) there are some significant differences between the two types of modules.
This is not an exhaustive list!
FONA 800 & 808 start at 1 (one) where as the FONA 3G starts at 0 (zero)
After sending an SMS using
AT+CMGS, the FONA 3G returns two sets of CRLF's that do not appear with the FONA 800 or 808.
If you request the # of SMS's on the SIM module with
AT+CPMS? the FONA 800 & 808 reply starts with
+CPMS: "SM_P" whereas the 3G starts with
The hanging up command is
ATH0 on the FONA 800 and 808 and
ATH on the FONA 3G. You'll also have to send the FONA 3G a
AT+CVHU=0 command to tell it to listen for the ATH command (it will say 'OK' but not actually hang up)
Battery voltage measurement with
AT+CBC returns a 4-digit voltage in mV on the FONA 800 and 808. On the FONA 3G it's a floating point number in volts.