"If isnull then" formula question

Hello!

I'm trying write a formula that has multiple 'if isnull' directives in it. 

Like this:

if isnull {Billing_Data22.A}

then {Billing_Data22.B}

if isnull {Billing_Data22.B}

then {Billing_Data22.C}

else {Billing_Data22.A}

I realize this formula doesn't work, but hopefully this conveys what I'm looking to accomplish.  If {A} isnull, then {B}, but if {B} also isnull, then {C}, else {A}.

Thanks!

Rich