Recently I had to check the existing passwords in sddc manager in our VCF 3.11 environment and found out there is a simple way. Here it is.
SSH into your SDDC Manager using vcf user and go to the root prompt using su command and use the below command:
root@sddcmgr01 [home/vcf]# lookup_passwords
Select any product and then you will have to provide the sddc secured user credentials which you provided at the time of deploying SDDC manager in the VCF environment. This credential is also used for the backup of SDDC Manager and NSX Components.
This way, you can get all the passwords for all the components controlled by SDDC Manager in VCF 3.x
NOTE/Disclaimer: I had to Blur/Pixelate certain components in my screenshots as they are in a live environment.