hey dude i'm not using any standard formula

just as u know that general term of the expansion of (x+y)^n is n!/r!s! x^r y^s  where r + s = n

so extending this to three terms general term for (x+y+z)^n is n!/r!s!t! x^r y^s z^t where r + s + t = n ,

this is very basic my friend!!!!!