No, it's not required for a L3 judge to be a HJ at a SJC, it's just normally expected. PTO's like the safety of knowing that their judge is a L3 judge since it means that chances are they have a solid understanding of the policies and rulings. It's a lot harder to take the L3 test than people realize, not only do you have to pass the test, but many UDE folks won't give a test to someone they feel isn't ready to take it due to one reason or another (for instance you may know the rulings well enough to ace it and they'll know that, but if you don't show strong leadership skills and a well versed knowledge of the policies then they may not even let you touch the test).
PTO's know this which is why they normally search out L3's for SJC events. Wil Loftis who did our SJC last year in Charlotte was only a L2 judge when he did it and we actually ahd John Williams on the floor for that weekend (John is and one of the first L3 judges) so it's always up to the PTO in the end.