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We present the first simultaneous photometric and spectroscopic investigationof a large set of RR Lyrae variables in a globular cluster. The radial-velocitydata presented comprise the largest sample of RVs of RR Lyrae stars everobtained. The target is M3; $BVI_{\mathrm{C}}$ time-series of 111 and $b$ fluxdata of further 64 RRab stars, and RV data of 79 RR Lyrae stars are published.Blazhko modulation of the light curves of 47 percent of the RRab stars aredetected. The mean value of the center-of-mass velocities of RR Lyrae stars is$-146.8$ km s$^{-1}$ with 4.52 km s$^{-1}$ standard deviation, which is in goodagreement with the results obtained for the red giants of the cluster. The${\Phi_{21}}^{\mathrm RV}$ phase difference of the RV curves of RRab stars isfound to be uniformly constant both for the M3 and for Galactic field RRabstars; no period or metallicity dependence of the ${\Phi_{21}}^{\mathrm RV}$ isdetected. The Baade-Wesselink distances of 26 non-Blazhko variables with thebest phase-coverage radial-velocity curves are determined; the correspondingdistance of the cluster, $10480\pm210$ pc, agrees with the previous literatureinformation. A quadratic formula for the $A_{\mathrm{puls}}-A_V$ relation ofRRab stars is given, which is valid for both OoI and OoII variables. We alsoshow that the $(V-I)_0$ of RRab stars measured at light minimum is perioddependent, there is at least 0.1 mag difference between the colours at minimumlight of the shortest- and longest-period variables.

Photometric and radial-velocity time-series of RR Lyrae stars in M3: analysis of single-mode variables