Three seconds is probably about the time it takes you to realise that you've got a nice image, think "Oh, that's a nice image", pay too much attention to it and ruin it thus. If you're starting out visualising, or otherwise aren't proficient, you'll notice that it's much harder to get vivid imagery when you're aware of it being vivid. Perhaps you've heard this advice before, perhaps even from me, but you should relax more during a session, focusing less on 'getting' and 'holding' good images, and more on just 'seeing' them more passively. Certainly it will help to space out a little bit and go with the flow, because, as demonstrated, if you become too aware of your visualisations they can be spoiled. Of course when (if) you improve you won't be bounded by that so much.