How to obtain the explicit solution of the following integral equation

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I'm considering the following integral equation $$ f(x,y,z)=x+\int_0^x\int_0^y\int_0^z f(u,v,w) dudvdw $$ It seems that $$ f(x,y,z)=\sum_{n\geq 0}\frac{x^{n+1} y^n z^n}{(n+1)!n!n!} $$ is the solution. How to transform it into a closed form instead of a series?