Printed out schedule and maybe (in both English as well as Chinese) a sign explaining your departure from T3 at 3.30am will definitely be a good idea to bring, even if you made OLCI already and are holding boarding passes (both to show to other passengers to let you queue jump as well as to show to staff to let you directly go in front, depending in queue length)

Unfortunately, in PEK, TWOV has been made more complicated. You'll first need to head for a special TWOV counter, where they'll check your schedule. If it's a no-brainer, like in your case, the check should be faster and smooth, but will still take a few minutes most probably.

After that, you can head to regular immigration.

So TWOV is (always) slower than if you're holding a regular Chinese visa, could be anything from a few minutes (no queue, boarding pass in hand, clear schedule) to up to 1.5hrs (my worst personal experience - luckily I'm back on a multiple entry now) at TWOV alone, but regular immigration lines were terrible that day too, so it "only" added approx 45min because in the end, after 6am more immigration staff arrived.

Anyway and spoken in general, not just your case, transfer between T2 and T3 (and vice versa) isn't fun, and if other options are available, I'd probably do them unless they're significantly more expensive.

If you're on a proper night-over (or several days stay) it doesn't matter, and TWOV, albeit now slightly slower in PEK, won't break any fun (and honestly is a great thing when comparing to rules in Schengen or the US) but for a quick transfer between Terminals, it's not something I'd fancy a non-seasoned traveller to do.

Transfer within T3 works out (relatively) smooth, last two transits I did this way were 40min resp. 55min from "doors open" to "crack open a beer in the lounge", which, while not great, is acceptable. But that only requires tons of walking, showing your passport for a quick immigration check (but no stamps), and getting a security check, but nothing more.