While 3GP is a video format, you can "convert" text into a 3GP file by creating a video where the text is displayed on the screen.
If you are looking to "post" text within a 3GP file or convert text into a 3GP format, 📱 Text in 3GP Files
You can use simple video editing software to place text over a solid background and export the project as a .3gp file.
3GP files can contain timed text (subtitles) alongside video and audio. This is usually stored in the 3GPP Timed Text format.
3GP is a multimedia container format primarily used on 3G mobile phones, but it can also be used on some 2G and 4G phones. It is a simplified version of the MP4 format and was designed to accommodate the lower bandwidth and storage capacities of older mobile devices.