You can not install a CFW on 11.3. 11.3 patched out all known exploits, including DSiware downgrade and hardmod downgrade. These were patched out because they added a NATIVE_FIRM requirement to the homemenu.
You can upgrade to 11.3 if you are on a CFW because your CFW (Unless it's brickwait) will protect your FIRM0/1, thus protecting your A9LH install. They will also protect your other entrypoints if you are using an emuNAND, but there's no reason to keep using those when A9LH is so much better.
Seriously this ^
Every new system on the market should still be on 11.2. It takes 10 weeks for shipments to leave China. From there they sit in warehouses until they are needed. You have plenty of time to get a brand new and exploitable system