Rarely, .dat files are MPEG-1 videos and these can be converted to other multimedia formats, including 3GPP format, but both of these file suffixes are rarely used for these formats and MPG and 3GP are the usually choice.
Plenty of multimedia converters support MPEG format, just not every recognize it as .dat file, so renaming dat to mpg before you attempt dat to 3gpp conversion might be helpful. You can find more software picks for this conversion under the mpg to 3gp conversion.