So, you have Flash Drives A, B, C. Windows resides on A, and drive B is NTFS formatted for overflow data. You are adding drive C and you want to put ubuntu on it so that you can dual-boot.
It really comes down to the size of your SSD's. If A, B, and C are all 1TB each, make 500GB on A windows and 500GB on A ubuntu. If you have a small 128GB SSD for A, you need about 30GB for Linux 20.04 so this might be an issue for windows. In such a situation, I would upgrade to a bigger SSD. 1TB has done me well on my dual-boot with 500 windows, 500 ubuntu.
First, make a backup with Macrium Reflect or similar of A. Make sure you have your windows backup USB. You'll thank me later.
For a dual boot system, you are going to need to put your grub bootloader on A and the ubuntu root on A.
If you have unlimted storage, I would just let the program install it on whatever free portion of your A drive you freed up
If there is a way around this, I am unaware. So run diskmgmt.msc and shrink A by at least 30GB to make space for /root. Run ubuntu on your flash drive, and when it goes into how do you want to install ubuntu, choose "something else". That will load up the Gparted editor in the installer and you will need to format the partitions you are making /root as ext4.
One thing you might want to consider is making C your /home partition, particularly if you have a lot of stuff to store. For a smaller SSD on A (256GB or so), it would make sense. Format it ext4 and proceed with the install.
Gparted is like any partition editor, prone to unintended consequences. Hence the macrium.
See: Questions about Ubuntu 18.04 LTS Install / partition sizing on SSD / HDD system.
sudo fdisk --list
please edit your question for output. – nobody Jul 22 '20 at 11:11