here i am writing the ISR for your code which might help you out check it
uchar i; // declare i as a gloabl unsigned char
ISR(USART_RXC_vect)
{
i++;
if(i = =13)
{
mybyte=UDR;
if(mybyte[1]= = 65`)
LED=(1<<LED0);
if(mybyte[1]= =70)
LED=(1<<LED1);
if(mybyte[1]= = 51)
LED=(1<<LED3);
_delay_ms(1000);
LED=0;
i=0;
}
else
mybyte=UDR;
} /* end of ISR*/
checkout and let me know weather it has worked???