I think he isn’t in the Z tier anymore as his health stats and defense stats are very low so he can’t really take many hits unlike other more well rounded characters like ssj Bardock or super Gogeta. This is ssj3 Goku weakness and he is not able to overwhelm opponents without drawbacks. Most characters in the Z tier are usually low(medium) risk, high reward members and are essentially broken characters. I believe ssj 3 goku is a rewarding character if you know how to use him but he is not broken by any means.
Why isn't ss3 goku in z tier? I get that he's a high risk reward character but the reward greatly outstrips the risk. He is also optimal on a team without broly anyways.