Well my experience is slightly similar, with the same Uworld average. Ended up getting a 400 on NBME 4 less than a week before my exam, which is also barely passing (looked up a score converter and my other NBMEs for Step 1; not really accurate, but the best indicator I had). Ended up scoring a 229 on the real deal. The reason why the Uworld and NBME might not be completely accurate for someone is because they don't include CCS.
The days leading up to my Step 3 Day 1 and before Day 2, I spent some time studying CCS hardcore. That's my advice. And, if you can, try to finish up the Uworld! Or get close enough to it.
I'm also under the notion (false or not lol) that if you did well on the previous Steps, you can surely at least PASS Step 3.