XPS 8950, adding two 2.5 SDDs in addition to 3.5 HDD

 If the operating system is not on the HDD and you are replacing it with another 3.5 inch HDD, then yes. Take the old one out and put the new one in. https://www.windowscentral.com/how-manage-storage-devices-windows-11 

To be clear, I am interested in moving only my two 2.5 SATA SDDs from my old computer to the new 8950.  The new 8950 would come with an NVME SDD and a 3.5 HDD.

I have checked the manual and did not find an answer to this question: How many SATA III ports and power ports are there in the 8950?  I would need 3.  The new 3.5 HDD on one side and two 2.5 SDDs stacked in the empty HDD slot on the other side.  

I was coming here today to start a thread about the 8950 HDD but I will ask here, I have a 2 TB ssd and 1 tb HDD, I want to get a larger HDD, is it as simple as just taking the old one out and replacing it with the newer one and that is it?

You can check the XPS-8950 Owners and Service manuals PDF (for this question and any others you might have).

I suggest you use this opportunity to depreciate any remaining 3.5-in spinning HDDs. If you just must keep them a little longer, they can be installed in a USB-3 External Enclosure.

Only install SSDs inside your nice-new XPS-8950. You can use m.2-NVMe SSDs as well as 2.5inch SATA-3/600 SSDs. With an appropriate tray, you can install TWO 2.5-inch SSD inside a single 3.5-inch bay if needed.

I found thesde helpful diagrams that confirm there are a total of 3 SATAIII slots in the bottom-right of the motherboard as well as two power connectors at each HDD bay so it appears that I can have a 3.5 HDD in one bay and two 2.5 SDDs in the other bay.  Plus there's the two slots for NVME drives.  Perfect.  

Hopefully my generic 2.5 to 3.5 cage that holds two SDDs in a 3.5 HDD slot will work.  If not, I'll find something else.  Likewise if I need different SATA cords they should be easy enough to find.  

I did exactly the same as you wish for.  I bought a blue cabby which is 3.5-inch in size but can hold 2 x 2.5-inch HDD back and forth. I have 2x Crucial MX500 2TB SSD in that cabby.   The power cable provided by DELL has two connectors so no issue. However, you have to discard DELL-supplied right-angle-shape SATA connector as it will conflict with the cabby frame.  Buy yourself two 40-cm flat-shape SATA connector will do the trick.  I moved the 3.5-inch HDD to the far bay with the DELL-supplied (? not exactly remember, I may buy it by myself) 3.5-inch cabby.  Power cable is available with two connectors. Again buy yourself the 60-cm SATA connector WITH the right-angle-shape to the HDD otherwise the case cannot be closed properly.  Of course, if you choose to put 3.5-inch HDD in the closer bay you can use the DELL-supplied right-angle-shape SATA connector.

I recently purchased Dell caddy #0GW1T5 for my XPS 8950. It holds 2-2.5" SSD's and fits perfectly in either of the 3.5" HDD cages. The best part is that NO screws are needed to mount the 2.5" SSD's.