The use of anything but properly designed sunglasses is very foolish and poses great risks to your long term sight, and maybe for reasons that many people do not wholly appreciate.
First of all, let's write down what unpolarised light is. We choose two basis polarisation states: let's go with left and right polarised light in this case since you say that 3D goggles filter these (I actually don't know what 3D goggles do: since I am blind in one eye I can't see 3D, so I've never given it much thought because I have no intuitive grasp of what 3D sight "looks like"). A general polarisation state can be represented by a vector of two complex numbers $E_+$ and $E_-$ standing for the amplitudes of the left and right components of the light and unpolarised light is where they constants vary swiftly with time: not too fast relative to the light's frequency but still fast enough that over any time period long enough to get a possibly harmful dose of light in the eye. Therefore, for the purposes of this argument we can think of the average total intensity of the light as made up of the average separate intensities of the two polarisations and, for unpolarised light, the averages of the two basis state intensities are equal. The total light intensity is thus $|E_+|^2 + |E_-|^2$ and once you knock out one polarisation state contribution with your 3D glasses, you block half the light. This result works for ANY choice of basis state pairs, so the same conclusion arises from the use of linearly polarising sunglasses. I'm not sure what you're thinking of when you say "3D glasses filter out a large proportion", but I'm guessing that you're thinking it's much more than one half. It has to let something through, otherwise the glasses would be blank! So half the UV light is still getting through your 3D glasses and this is well enough to cause long term damage that can be prevented by proper sunglasses designed to filter UV. That's assuming that the 3D glasses still work as circular polarisation filters im UV, which, as you say, is not certain.
As you rightly point out, the safety benefits of sunglasses are wholly owing to their working as ultraviolet filters. There are two risks to the eye from intense light:
- The first is the thermal risk, where there is too much energy getting into the pupil and the local heating at the focal disk on the retina destroys the retinal tissue. Contrary to commonly held beliefs, the damage risk from this effect in normal sunlight conditions for a HEALTHY retina is minimal - EVEN if you like migraines enough to stare straight at the Sun continuously! (there are exceptions for retinal disease or dysfunction - so it's not a good idea to test this assertion out). With our pupils shrunken to their smallest size (about 1mm diamter) at noon on a clear day, staring at the Sun will deliver a heat dose of about 1 mW to the retina. This heat dose is WELL within the capabilities of a healthy retina to dump: the retina is superbly well envasculated and healthy blood flows carry off even this level of heat with almost no chance of any damage. Interestingly, laser safety standards for longer wavelength visible light set the intrinsically safe level at about the same level we experience when staring staight at the Sun. You will of course be swiftly temporarily blinded by such staring though as the ATP energy reserves of your cone and rod cells are drained owing to overstimulation, so we have a strong phsychological aversion to such staring - from an evolutionary standpoint it is not good for either predators or prey (we are both) to be blinded for minutes at a time! As an aside, the danger of looking at a total solar eclipse is that the pupil only responds to average light levels and swells to 50 times its area (7mm diameter) in the near-darkness of a Solar eclipse: you are therefore at significant thermal eye damage risk if look at the "diamond ring" and following bright stages of an eclipse after totality, when you can easily get thermally highly dangerous doses of 20mW or higher.
- Here's the main issue with sunglasses. Longer wavelength UV will cause sunburn of the retina at even low but constant levels. Shorter wavelength light begets phototoxicity: this arises when the light is energetic enough (photons of short enough wavelength) to knock outer shell electrons off and bring about chemical change and thus real cellular damage (even cellular nuclear damage, with its attendant cancer risk) within the retina. You wear sunglasses to avoid the painful longer UV wavelength sunburn that you'll inevitably get at chronic exposure to even low level sunlight and avoid the medium and long term risks of cataracts or worse (even eye cancer) that arise from chronic exposure to even low levels of shorter wavelength UV. The factor of one half attenuation I cited above for polarising filters does very little if any good in mitigating these risks.
So forget about sunglasses being useful for their general power attenuation effects. The power alone of sunlight in normal conditions (i.e. not an eclipse) is an almost nonexistent risk. It's all to do with phototoxicity and sunburn, so you need properly designed sunglasses that can attenuate the relevant UV bands by several orders of magnitude. You might get some relief from the discomfort of squinting in bright light, but as shown above, that discomfort is almost wholly psychological and as you point out the filters will likely raise the UV dose to your eyes owing to the swelling of pupils in low light levels. You can't feel the real damage proper sunglasses avoid till it's far too late.