Explain why the insertion of 3 nucleotides is less likely to result in a deleterious effect than an insertion?

As you probably know, a codon is a sequence of 3 nucleotides along an mRNA molecule.  Each codon specifies a particular amino acid.  The insertion of 3 nucleotides into an mRNA strand will more than likely result in the addition of another amino acid, which may not necessarily cause any significant damage to the polypeptide (silent mutation).  However, the insertion of one base into the mRNA strand will result in a frameshift muation, in which the reading of the code is shifted by one base.  I will try to give an example.

Consider the following sentence:

THE FAT CAT ATE THE RAT

The insertion of a base will result in the following:

THE FAT ACA TAT ETH ERA T

Here, the insertion of "A" into the sentence represents a nonsense mutation.  This can result in the premature termination of the polypeptide.  Hence, the polypeptide will be shorter than the one encoded by the normal gene.

I hope this makes sense!