There isn't a file system on the PSoC 3 system that you are using, so the whole concept of File IO is not defined. Since you are interested in writing to a file, the most likely alternative is to use a communication interface to send data out of the device. The simpliest is likely the UART.
So if you put a UART into your design, then you can write to that UART and have a PC or other device recieve the data.
If you want standard C formatting routines, those are supported by the Keil compiler. For example you can use the sprintf function to create a string and then use the UART_PutString() function to send that string on the UART interface. Make sure to include the stdio.h header file as well:
You can refer to the help documentation for the Keil compiler (from Creator Help->Documentation->Keil). From there search on stdio. You will find that Keil also has the concept of printf, but that is implementation dependent and needs an interface to use. For PSoC 3 you should use sprintf and then pass that string to whatever commnunications interface that you would like to use.