I am not sure that you can do this without a protocol driver for modbus. But with TCPIP it may be possible to directly address the device.
As far as I can remember, modbus requires an integer "command" and returns an integer value based on the reolution of the A/D converter.
Example: 20°C with an 16 bit converter (65536).
resolution of DAQ x °C / Maximum measurable Temperature = 65536 x 20 /1100 = 1191.56 Modbus would transmit 1191 and then you convert it back.
Establish communication with your device, telling it to wait for a command. Then send the integer 40001 (4 is the command to read a register, 0 is the register in this case). Not sure why you want to read using the command 3! Careful if the device is 32 bit, as the result will be in two registers, high bit and low bit for a floating point value. You will probably need to scale the resulting value as well. Never tried this with android.
I have no idea what library you can use but try searching for microprocessor control here in the forum. People control all sorts of things here!!
If you get this working, I would be very interested in how. Good luck.