Where can I find the genotype frequencies for the SNP's that are tested at 23andMe, Ancestry and FTDNA/MyHeritage?

gnomad is probably the most comprehensive allele frequency database that exists at the moment.

Depending on how many variants you need to query, you can either manually input them into the search bar, or if you have a lot more, then you can also directly download the vcfs under the downloads tab.

The default for V2 gnomad set is on the gr37 build.

The largest number of samples is at the dbSNP database. To view frequencies of a single rsID in a web browser, for example, rs11950646, use the following URL:

https://www.ncbi.nlm.nih.gov/snp/rs11950646#frequency_tab

The dbSNP has more samples to calculate the frequency than gnomad, at least for this rsID.

If you need to automatically process multiple rsIDs, as you said, for the whole genotype file that you got from the companies like 23andMe, you can download a VCF with allele frequency data from https://ftp.ncbi.nih.gov/snp/population_frequency/latest_release/ and write a script to automatically process the data and make the output in a format you want. If you only need frequency of some rsIDs within a single chromosome, you can download the whole database for that chromosome from https://ftp.ncbi.nih.gov/snp/latest_release/JSON/ and will find the data in the JSON files, each file for each chromosome (plus the mtDNA). Look for the "frequency" field.

If you are not engaged to write scripts, and just wish to interactively browse various rsIDs in context of a frequency, you can upload your genotype file to selfdecode.com - it is a convenient, albeit a paid service.