Can Decimation be used to improve the SNR of a signal?
Decimation basically means downsampling or re-sampling the signal using a lower sample rate, see the article on Wikipedia.

There are several ways to do the decimation and it depends on the chosen method and the type of signal what effect decimation has.

When as in your example, the 1 out of 3 samples is kept and that is combined with lowpass filtering (the filtering might be needed to avoid aliasing) then I can understand how that could increase the SNR. If the noise content is mainly in higher frequencies (higher than the signal's frequencies) then the noise will be filtered out while the signal should remain mostly unaffected.

A different well known method for downsampling is to take the average value of multiple samples. Then additional filtering might not be needed. Note that the averaging does introduce a low-pass filtering effect but in the digital domain. Again if the noise is mainly at high frequencies part of that will be filtered out which improves the SNR.