Are your variables (double) integer, floating or fixed point?
Essentially its just
Add R1 to R2,
Add R3 to result,
Divide by three.
implementation will be dependent on what CPU you use, whether you wish for it to be a function call or inline and whether you wish for a fixed length of variables or dynamic length.
Assembly language memnonics are different for different CPU's
Common Microcontroller CPU's are PIC and AVR, but could be anything else. Assembly language for just these two reads quite different and won't work on each other.
Even the assembly language is different for 8051 and 8085, 8086 which are designed by same firm Intel...
Specify the CPU or microcontroller you are using...