So, in near future OBS will change it's interface and allow basically only to select one repository for "Code 15-3". You can decide to include Backports or Leap packages on top of SLE 15 SP3 as build and install dependencies, but no matter how you configure it as OBS user, the resulting repository name should have the same name.
The big question is, what should this repository name be?
Just make it short.
openSUSE_Leap_15.3 and friends always make me cringe. I always manually named my repos "Factory", "TW" and "Leap_15.2" etc. but it's additional hassle, especially as you cannot use "zypper ar obs:///..." easily with them.
What about just naming it
15.3
People need to understand that they can use it on openSUSE Leap 15.3 or on any SUSE SLE 15 SP3 installation.
Everyone out there calls that beast SLES15.3 anyway, so it would fit naturally.
very good point, however I wouldn't use 15.3 alone, you might at some point end up with collisions with some other distro (yes, there is still quite some room, but never forget the factor of time, and OBS is quite around now) Then how about just simply: SUSE_15.3