is there any 24pin to 8 pin connector adapter for the OptiPlex 3000?

no known adapter exists but you can upgrade to higher power psu for Optiplex 7000/5000 which also work for 3000.  do you have a tower or SFF?

@redxps630​  I have the SFF I'm currently trying to upgrade the PSU and install a low profile 4060. will those psu work with the 3000 sff also?

by default the 3000 series SFF chassis does not support dual slot width LP gpu unless you install PSU outside case

@redxps630​  I was planning on doing that since it will not fit with the PSU inside. so i shouldn't have any problems with those PSUs and also using the rtx4060, correct?

@redxps630​ thank you so much for all the help! do you know if I will have too buy extra cables to power up the GPU? I am kinda new to this.

7000 tower 500w psu has 8 and 6 pin PCIe black connector

https://www.ebay.com/itm/364440153519

rtx 4060 LP needs one 8 pin or one 6 pin.  no need for extra cable.

https://www.gigabyte.com/Graphics-Card/GV-N4060OC-8GL/sp#sp

https://www.asus.com/us/motherboards-components/graphics-cards/asus/rtx4060-o8g-lp-brk/techspec/

@redxps630​  is there a post or a test that someone has done this before? I don't wanna fry my system if it does not work.

I don’t have a direct example of using Optiplex 7000 500w psu in Optiplex 3000 sff, psu outside case.  I am 100% sure the psu between these two models are interchangeable.

let me ask mod to support this claim

@DELL-Chris M 

can you please let us know the DP/N of oem psu for Optiplex 7000 tower, 7000 SFF, 3000 tower, and 3000 SFF?

if mod confirms the 240w and 300w psu of these models are interchangeable then the higher power 400/500w oem psu are compatible too to power up motherboard safely.

https://www.delltechnologies.com/asset/en-us/products/desktops-and-all-in-ones/technical-support/optiplex-7000-towers-spec-sheet.pdf

https://katalog.atcomp.cz/katalog/D-D-3000U-501/optiplex-3000-towers-spec-sheet.pdf

@redxps630​ 

XYD3F 400W Liteon Optiplex 7000 SFF/Tower
CW96Y 500W Liteon Optiplex 7000 SFF/Tower
VFFKJ 500W Liteon Optiplex 7000 SFF/Tower

No go on Optiplex 3000 SFF/Tower.

@DELL-Chris M 

Thank you for those P/N.

can you please let us know the part number of 300w oem psu for 7000 SFF and 3000 SFF ?

@redxps630​ From what Crhis said will it be possible or not? thanks!

3000 SFF =
3XY3J 300W Chicony
6R2MV 300W Delta
HD2W8 300W Acbel
MJXT6 300W LiteOn

7000 SFF =
3XY3J 300W Chicony
6R2MV 300W Delta
HD2W8 300W Acbel
MJXT6 300W LiteOn
CNHDM 300W Liteon

What Chris is saying is Dell has never validated the 500/400w psu for 7000 tower in the 3000.  This makes sense because (1) the high power psu form factor being wide is not physically c/w SFF chassis  (2) Dell is not responsible if user installs Dell psu off-label.  If you use 7000 psu in 3000, you are going on an adventure on your own.  
This on the other hand does not mean Dell has done test of the high power psu in 3000 and found it is no go.  Dell has not done this test at all.  This is an off label area.

I presumed that these model psu are electrically compatible, which is now confirmed by Chris.  As Chris stated, the 7000 and 3000 SFF use identical 300w psu.  Now look at Chris first answer: the 400/500w psu are the same for 7000 SFF/Tower.  Well that is a Dell statement that 7000 tower and SFF psu are electrically interchangeable.  
so user can extrapolate:

7000 Tower/SFF can use same 400/500w psu

therefore they can use same 300w psu too

3000 and 7000 SFF can use same 300w psu

therefore 3000 SFF can use 400w psu for 7000 too.