3rd SB mewtwo worth power up?

Trade to someone else who don't have a Shadow Ball Mewtwo and gain something else good back, it isn't going to be that good with only 11 attack IV, considering you have other better ones.

The important question is do you feel like you need it? If not, then it's not worth powering up right now.
SB Mewtwo is a top counter for Mewtwo raids right now, could be a vital attacker in future raids or a valuable trading asset. No need to rush any decisions.

Do you really need it to improve your Mewtwo team?

I got carried away when SB became legacy and powered my 3rd (10 attack). Only because it was weather boosted and brought it up to level 30.
I only had two dark tyranitar. I will use it against Mewtwo for a whole month, so I think it's worth it.

Depends one what you need. If you have a large number of psychic counters I'd say no, but if you're like me and dont have a good number of them (1 tyrantiar, one houndoom, scizor, and 3 SB mewtwo with only 1 being powered up), power it up.

As others have said, if you’re lacking a good anti-psychic/ghost team then sure. If you’re not, you might as well save the dust and candy. You could try to find someone else with a shadow ball Mewtwo and swap them hoping for a lucky.

That said, you’ll definitely get a lot of use out of Mewtwo with shadow ball. We’ve got a month of Mewtwo raids, Deoxys coming in EX raids, Cresselia, Lake Guardians, and Giratina raids in gen 4, so it’s a good investment.