A frame shift mutation that results in the insertion of 3 nucleotides doesn’t reading frame of the downstream codons and proteins they will be made into.
A mutation that inserts a single nucleotide ultimately will change the reading frame and all of the codons (3 nucleotides) downstream of that insertion will make a different protein than they were suppose to or a stop codon could be made which will completely stop that protein from being made.
Example: AUG UAU ACU CAC
The example above will encode for the following proteins.
met, Tyr, Thr, His
3 nucleotide insertion in this example could be:
AUG UAU AAU ACU CAC
met, Tyr, Asn, Thr, His
1 nucleotide insertion mutation:
Example: AUG UAA U ACU CAC
Met, stop codon
Nucleotides after the A insertion are not made into proteins.