Try third party tool. I use this video converter which can help convert 3GP, mts, mxf files to apple prores MOV which FCP X can recognize. Many people said FCP can read almost all the video formats but under my experiences, it is not true. I have VRO and MXF files which are both beyond its ability.
3gp is not supported with FCP X, and you should first convert your 3gps to mp4, mov, or avi, then import the converted mp4, mov, or avi file o FCP X.
I am always using third party tool "Total Video Converter" from App store to convert my Youtube FLV files to iPhone, so great. And you will search it easily on App store.
But if your OS is lower than 10.6.6, you can search "Total Video Converter etinysoft" , and the first result is the same app as that on App store.