To adequately answer this question, we need to know the spectral class of all of the stars in this system, however, the effects on humans of each of the spectral classes individually can be described:
Class O, B - blue stars. These stars have a very high UV output, as well as being very bright for their size. Humans exposed to their light would be quickly sunburned, and unlike our sun, just having such a sun in your field of view for more than a few seconds could cause temporary retinal afterimages. Longer exposure could cause blindness without sufficient protection.
Class A, F - white & yellow/white stars. These stars are not quite as bright, but they still produce more UV than our sun, though less than O and B stars. Sunburn would still be an issue, though blindness would not be much of an issue.
Class G - yellow stars. Just like our sun. We know what the effects of such a star are from personal experience.
Class K - orange stars. Not much UV at all. Dark skinned people could suffer from Vitamin D deficiencies if not provided in their diet even if they go about practically naked. Light skinned people might get enough UV to produce Vitamin D if they don't wear much clothing. Sunburn is not an issue.
Class M - red stars. No UV to speak of. Everyone has to gain Vitamin D from their diet, since they won't be able to produce it from UV exposure.
The effects of having several of these stars in the same system would be cumulative - blue stars could cause blindness and sunburn, but any UV-producing star would compensate for the lack of UV from orange and red stars.
Size doesn't matter, it is only the colour that matters. Blue stars produce most of their radiation in the UV part of the spectrum, and red stars produce most of their light in the IR part of the spectrum.
Given equal visible light luminosity, the red star would produce no UV, and thus sunbathing in its light would cause no sunburn regardless of the time spent in its light. This would be the warmest part of the day, given its high IR output.
The yellow star would produce enough UV to prevent vitamin D deficiencies, as would the blue star.
The blue star itself, while not producing more visible light in total, would concentrate its light into a smaller point in the sky, and would appear much brighter, easily causing retinal afterimages like looking at a welding arc. Also, it would cause sunburn quite quickly, in a matter of minutes. This would be the coolest part of the day due to the lower levels of IR.
Vitamin D deficiency would definitely not be a problem unless people hid from the blue and the yellow suns altogether.
Atmospheric ozone could mitigate UV exposure to some degree, but it is unlikely that such an effect would be much greater than that of our own ozone layer, and could well be less.