I have 3 partitions (mmcblk0p1, mmcblk0p2, mmcblk0p3) on a disk.( mmcblk0 )
I am using uBoot as a bootloader.
I have 2 same variants of Debian 9 operating system image flashed on 2 partitions ( mmcblk0p2, mmcblk0p3)
Partition 1 ( mmcblk0p1 ) is the partition Where I am storing uEnv.txt ( For deciding which partition to boot from )
I am unable to boot from the Partition 3 (mmcblk0p3) When I give root=mmcblk0p3 and other configs in the uEnv.txt. It is booting from the mmcblk0p2 ( According to my understanding it should boot from mmcblk0p2).
To find out if I was doing right I configured 2 partitions only ( mmcblk0p1, mmcblk0p2 ) both with the Debian 9 OS image. When I configured the 1st Partition ( mmcblk0p1 ) to root=mmcblk0p2 and other configs in the uEnv.txt. I was able to boot to the 2nd partition (mmcblk0p2).
I am confused Why it's not booting to the partition I give when I have 3 partitions?
But works perfectly fine when I have 2 partitions and boots to whichever the partition I give.
Can anyone help me? I'm a pretty newbie to uboot and Linux ( Debian )

According to the configuration page, you should have: root=/dev/mmcblk0p3

Also, the /boot/grub.cfg file in each OS is likely to be configured to boot a partition based on it's UUID. I'm guessing the two OSs are from the same image file you extracted yesterday from the disk image, so...each partition has the same UUID, as such, grub will be unpredictable.
You can make it predictable by editing /etc/default/grub in both OSs to disable the use of UUID which will have grub looking for /dev/mmcblk0p2 or /dev/mmcblk0p3. By simply editing the line in the example below to remove the hash (#) at the beginning of the line. If that line don't exist in your /etc/default/grub, paste it in there. After you edit this file you need to issue command as root or sudo: update-grub. Then boot into the other OS and edit it's /etc/default/grub file the same way and issue the update-grub command again.

Thank you for the response Brains

Sorry for that was a typing mistake in the above post. I have already configured by adding root=/dev/mmcblk0p3 in my uEnv.txt

Yes the two OSs are from the same image file ( But it is going to change in the future. One OS can be from a different image than the other one. )

I am not using grub. I am using uboot as a bootloader. ( Will the configuration be same for both as you told above?)

I couldn't find any uboot configuration under /etc/default. In my case, there is a uEnv.txt under /boot and as explained in the previous question if I have 2 partitions and I configured uEnv.txt to boot the second partition it's working.

But this is failing when I have 3 configurations as explained in the above Question.

Not sure, I've never dabbled with uBoot loader, I just added some information as it might be relevant. If you were to use Grub2 as the boot loader, then you would have to configure Grub to look for device rather than UUID when there are multiple matching partition UUIDs.

Okay got the solution needed some more configuration files in the partition to add for it to set the boot priority.
Thanks for the response.