Hi,
I think LDRs are very sensitive to any dispersed light and "messy" anyway. You shouldn't need to but you could add a comparator to sense LDR voltage rise/fall to trigger the timer.
If the 3 seconds on time is not truly necessary, you could also take the timing components off the timer, tie 2 and 6 together or use them separately and use them as a window comparator. If there are hands present, output on; if there are no hands present, output off. So long as 1/3 and 2/3 Vcc are acceptable on/off levels (and you can tweak those levels a fair amount without even using pin 5), it's a simple option.
I was messing around with this the other day:
**broken link removed**
Your circuit looks like the positioning/location of the IR LED and LDR are very important to consider and test in the real world at an early stage to get meaningful results.