3AC/TAC is something fundamentally different than SSA. Sure, both somehow superficially constrain how you write variable assignment statements - if your intermediate representation contains such things and is a list of instructions. 3AC means that your "instructions" have <=3 operands, SSA means each "syntactic" variable is assigned at most once, that is: they are not a variable anymore. In fact, SSA transformation makes an imperative program referentially transparent (excluding any loads/stores from/to memory) and thus makes it a pure functional program (excluding memory). You can see a program in SSA form simply as a functional program where each basic block is a (nested) function and each PHI is a parameter of the function. SSA as a list of instructions/statements as in 3AC is a red herring and doesn't have any relation to why it's used in compilers/for static analysis. SSA is not a variant of 3AC, also because SSA transformation of 3AC requires the same algorithms as SSA transformation of any other common imperative IR does.
In fact SSA makes no constraints on the number of operands in an instruction/operation what so ever.
In fact SSA makes no constraints on the number of operands in an instruction/operation what so ever.