How long did you have this problem?
Check in setting that 3G is enabled (not 2G only).
You have a pretty old firmware, you can try updating to 188.8.131.52 or later.
Make sure your provider does provide you 3G and it is working with other phones using the same sim.
Not sure if it is different with flyme 3.
Download desired firmware from https://firmwares.meizufans.eu
Two ways of installing your downloaded update.zip. Either clicking on it with the default file explorer or through the stock recovery. I would suggest you to tick cleardata if you are updating to flyme 4 or 5.
To access recovery, power on the phone with volume up and power button. To install, connect the phone to PC and copy the update.zip file (do not extract) and then tick cleardata and start to install.