Wherever I searched about unification in first-order logic (FOL) expressions, I could only find it as the unification of "two different logical atomic expressions identical by finding a substitution". So, I want to ask if it is possible to unify 3 different sets of FOLs.
For eg: Expression {p(b, X, f(g(Z))) and p(Z, f(Y), f(Y))} can be unified into { p(b, f(g(b)), f(g(b)); p(b, f(g(b)), f(g(b))} and the Most General Unifier(MGU) would be { b/Z, f(Y) /X, g(b) /Y}.
But if we add a 3rd term to the expression, say {(, (), ), ((), (),), (, (), )}, can we unify the FOLs? If yes, what would be the MGU?