According to Wikipedia, "Many amateur astronomers consider it one of the finest northern globular clusters, following only Messier 13. . . . This cluster is one of the largest and brightest, and is made up of around 500,000 stars. It is estimated to be 8 billion years old. It is located at a distance of about 33,900 light-years away from Earth.[citation needed. . . It contains 274 known variable stars; by far the highest number found in any globular cluster."
Having recently read "The Glass Universe" by Dava Sobel which documents the extraordinary work of the female 'human computers' at Harvard University cataloguing stars, their positions, their spectra and their significant contributions to astronomy and especially variable stars and their brightness/periodicty relationship, it is rather pleasing to capture M3 with its abundant stars and variables.
Captured with my Takahashi FSQ106 reduced to F3.6; 10 Micron GM1000HPS mount; QSI683wsg-8; Astrodon LRGB filters; comprising 15 x 300s Luminance and 8 x 300s each of RGB, total integration 3hrs 25m. May 2017.