FX3 in peripheral mode supports SuperSpeed,HighSpeed and FullSpeed. Low Speed mode is not supported when FX3 is used as a peripheral. Whereas, Low Speed mode is supported when FX3 is used as a Host. (Ref: Section 6.2 of FX3 Technical Reference Manual)
You can refer cyfxbulkdscr.c file of cyfxbulksrcsink example project (C:\Program Files (x86)\Cypress\EZ-USB FX3 SDK\1.3\firmware\basic_examples) for the Full Speed configuration descriptor when FX3 is used as a peripheral.