We report on our ongoing project "Statistical studies of HII regions in the nearby extra-galaxies". In this report we present the overview of our detailed study of warm dust in the nearby Galaxy NGC 4321 (M100). We measure the flux values in the 4 Spitzer-IRAC bands of some 275 HII regions in M100. In addition, we also present our new measurements the flux values in the 4 Spitzer-IRAC bands of a complete sample of 74 isolated luminous HII regions in NGC 4736 and 160 regions in NGC 4254.We study the relations between the Hα luminosity and the near-IR luminosity and temperature of HII regions in M100, in NGC4736 and NGC4254. We estimate the near-IR luminosities and compare them with the Hα luminosities from the literature. We are comparing the results obtained from these three galaxies. We find a linear relation between the Ha luminosity and the IRAC luminosity for the HII regions, but no apparent relation between the luminosity and the colour temperature of the regions both in M00 and in NGC 4736 and (most likely in NGC 4254). The color temperatures of regions especially in M100 are confined to a surprised narrow range, with a small fraction forming a higher temperature tail to the distribution. We support one of out of three proposed scenarios to account for these results.