Unfortunately, there was no “standard” light for the RV-3 tail - there was no provision for one, so whoever built your airplane did a custom installation. However, I am pretty sure that if you contact Whelen, they would have an LED replacement for that model light that you could adapt.
Paul F. Dye
Editor at Large - KITPLANES Magazine
RV-8 - N188PD
RV-6 (By Marriage) - N164MS - "Mikey"
RV-3B - N13PL - "Tsamsiyu"
A&P, EAA Tech Counselor/Flight Advisor
Dayton Valley Airpark (A34)