An optical study of the HII region populations evident in three galaxies, M101, M51 and NGC 4449, has been made. Using narrow-band filters, emission line imagery has been taken using a CCD focal-reducing camera, at wavelengths covering the emission from H α, H β, (O III) $\lambda$5007 and (S II) $\lambda\lambda$6716+6731. Using several identification techniques to spatially select the HII regions, emission line properties have been derived for 625 HII regions in M101, 465 in M51 and 163 in NGC 4449, making this the most complete study of its kind to date. Several trends have been discovered concerning the properties of the HII regions with radial position within their galaxy. M101 exhibits a large gradient in excitation, and oxygen abundance, as well as a gradient in the line-of-sight reddening. No positional variation in the derived ionization parameter for each HII region was found. Local variations in the effective collapse density for neutral gas have been detected for both M101 and M51. No such analysis was possible for NGC 4449 due to a lack of available data. M51 shows systematic emission variations only in the brightest cores of its largest HII regions, an effect attributed to a larger influence of the local ISM on the properties of the fainter, and more obscured, HII regions. M51 exhibits a spiral pattern that does not follow a single mathematical description, departing most dramatically at the corotation radius. A variation in the evolutionary time from peak local compression to peak star formation with radius has been detected for one of the arms in the galaxy, but not the other. NGC 4449 displays no systematic variations in the derived emission properties of its HII region population. This is attributed to a star formation mechanism that is independent of the radial ordinate, contrasting with the spiral density wave mechanism dominant in spiral galaxies. Unprecedented deep CCD imagery of this galaxy is presented, revealing the complicated structure of ionized filaments between the HII regions. The emission properties of these filaments are studied.