PIC18F14K50 and USBin

[Darrel Taylor]

I send 3 bytes and hope that all of them would be sent into the buffer (array of 3). But really after the first received byte the program writes something to the other 2 buffer bytes and begins receiving next byte. Then adds to it 2 its own etc...

With CDC, you have no control over how many bytes are received at a time. That depends on the operating system of the PC.
You may get 3 bytes, or you may only get 1, you have to account for the number of bytes received.

Your belief that USBIN is writing characters that it didn't receive, is completely incorrect.
Those characters are already in the buffer array.
If you clear the array before doing the USBIN you will see the difference.

Read Post #2 again, which will fix your sample code.