is it worth to power up my shadow claw gengar even if its lvl3?

In a way I’d say it’s not worth it, mainly because I believe SC Gengar can be overrated. Not in DPS but in actual use, unless you’re doing really tight solos or duos it won’t make much of a difference to spend that much dust on one

Then again SC Gengar is something that isn’t guaranteed to come in the near future. It still is great. It’s more of a matter of how much you yourself wants to power it up. Some would argue it is on the move alone, I think you’d be better off powering up a few Gengars instead if you didn’t have any

I’m a live for the moment player, so I guess it depends on what you need at the moment. Are you trying to more actively participate short trainer teams for mewtwo or alakazam (presuming not able to want to break your back to solo)? If not, it does not have urgency.

Your dust level is not as such that you can blow through it and laugh it off, It seems just enough to power up one or two, so I would say save it and power up something that does/ will have urgency

It is worth it if you need it, if you already can win it regularly either with strong squad or plenty of trainer then you dont need one

In my case, i have to because i play with new or returning player in my area, we just make a grup of 10 trainer but rarely we raid with full team, mostly 5-6
I admit i use 2 acc with 2 device, i need it to carry my team. Both use 2-3 Gengar
Last raid, only 3 trainer and me show up at mewtwo raid in my area they are at 26,27,29 with me 40 and 38. They try to set up their counter but they simply dont have enough good counter yet.
Lucky for us, it is Mewtwo with PC and Psychic, the Gengars still can fire 1-2 shadow ball then i use 3 mewtwo with PC-Shadow Ball and rejoin with 6 dark TTar. Cannot blame them using Golem, Flareon, etc. They only have 1-2 TTar, some houndoom with non optimal moveset, Pinsir with half bug move and such.
Win with only 5 second left, so Gengar help in my case..
If you can regularly find a big group to raid, investing in TTar and some Houndoom is easier. Shouldnt be to hard even raid against Confusion- Focusblast one in a big group of trainer, same team manage to make it around 1/8 left when we failed in prevous raid earlier

Not overly. SC Gengar's over rated to the point of blind faith, its 'highest DPs against mewtwo' is really unobtainable because in practice is doesnt deliver unless its a perfect one which no one has. It goes down extremely easily to half mewtwo's movesets and its only real time to shine will be against Deoxys who is a massive joke of a raid boss to the point where an SC gengar will be beyond overkill for little benefit. If you're aims to three/four man legendaries than i'd say power it up, but if not doing that i'd say dont, there better stardust investments

I seriously regret spending my stardust on mine as it sees absolutely no use

Sort of off-topic, but I'm wondering how you have saved up that much stardust. Teach me your ways! Do you just not power up that many mons, or do you have a stardust farming strategy? I'm about to hit lvl 30 and only have about 50k stardust, but I like to power up my mons as I go.

There's not enough information to give an advise.
Gengar would be worth the investment in many situations.

How many SB Mewtwos do you have? If you have 8-10, you absolutely don't need SC/SB Gengar for Mewtwo raids. If you only have a few, then the SC/SB Gengar can fill out the focus blast crew. The other movesets should be easy with B/C Ttar.

And of course, this is a game. If a level 40 SC Gengar would give you pleasure, then go for it. Doesn't matter if it only gets off 2 SB against FB Mewtwo, if you like it, run with it.