Can one get something bought before the three weeks tailored during the nine days? Second, does it make a difference if it is for shabbas, or for week day use?
In all likelihood this is not allowed.
See shulchan aruch siman 551 siff 7. The Shulchan Aruch mentions an that one is not allowed to be misaken ( you'll soon see why I'm not translating this) new clothes or shoes from rosh chodesh Av.
The Ramma adds that we allow non Jewish workers to be misaken the items during the nine days so that they will be ready after the fast. However he adds we should limit this allowance as much as possible as it is no better than all other business which we limit in this period.
The Mishna Berurah addresses why we allow the non Jew to be misaken the clothing even though we don't allow them to wash our clothing during this time period as seen in siff 3. He explains that at this stage in the clothings construction, it does not as of yet belong to the Jew and is not referred to as the Jews clothing. Apparently, according to the M.B. the Ramma was only referring to the old system where a professional builder owns the product they are creating. This would probably still apply with a custom made suit where the tailor owns the actual material until you buy it. But this would have no bearing on a suit bought in a department store which is then brought to a tailor.
The Aruch HaShulchan #20 has a different approach to this problem. His solution is that since washing clothing was explicitly mentioned in the gemara as being not allowed we are more stringent and don't even allow a non Jew to perform this for us. This seems like a promising leniency, however, he goes on with the idea that the Ramma mentions to limit this as much as possible and adds his own stringency saying that the only time to allow the non Jew to be misaken it is for a mitzvah like chassan and kalla for which we would allow the non Jew to be misaken on Tisha Bi'av itself. He ends off saying the minhag where he lived is just that, to not give any work to a non Jew unless it is for a mitzvah.
I'll throw in the fact that old clothing can be fixed if it is necessary as quoted in the Shmuos Moshe section of the Shmaatsa DiMoshe from Rabbi Eider in the name of Reb Moshe Feinstein.
Piskei Teshuvos #29 quotes a number of earlier sources who seemingly allow all types of fixing of old clothing, buttons hems patches etc. He does not stipulate that one may only do this of it is necessary.