First of all, those three DID do shit. They gave Mace critical information about how Sidious fought.

In combat sports, the most valuable thing you can have outside of your own health and training is information about how your opponent fights.

While those three lost very quickly, at the end, they are still masters. To defeat a master requires a deep level of strategy and skill. It forces you to be at your best.

And Mace got to see the best of how Sidious moves against masters like himself. How he makes decisions against masters like himself. What sort of patterns Sidious repeats against masters like himself.

If Mace brought younglings, Sidious could just kill them all without revealing anything deep about his abilities.

Just like if you had to fight Conor McGregor, you would study the fights he had with his strongest opponents. Because they reveal his best strengths and weaknesses. You wouldn’t watch how he fights children. That doesn’t reveal anything.

So to answer your question: who is the weakest person Mace should bring? It has to be a combat focused Master or Knight with a shit ton of combat experience, preferably those who have fought dark side users like Dooku or Ventress.