The Chasam Sofer to parshas Nasso says (Toras Moshe I, ד"ה ושמו שמי) that it is known that even though the Ten Commandments contain all the 613 mitzvos (see Rashi and here), the three mitzvos that were first commanded are not included. These are (1) pru urvu/be fruitful and multiply (2) milah/to circumcise one's sons (3) not to eat from the gid hanasheh/sciatic nerve.
I personally have never heard of this idea. Who says it, besides the Chasam Sofer?
I checked to see if Rav Saadya Gaon, who lists how the 613 are included in the 10 in his poem known as Azharos (see link above), excludes these three. I also used Rav Perla's commentary on Rav Saadya Gaon's list of mitzvos.
Rav Perla points out (Aseh #69, Chelek 1 page 281) that Rav Saadya Gaon doesn't even count the mitzvah of pru urvu in his list of mitzvos, let alone put it in his Azharos. Rav Perlow however holds that Rav Saadya holds like the Rosh (in Kesubos Chapter 1), that there's no mitzvah of kiddushin. Rather, kiddushin is just a possible means to fulfill one's mitzvah of pru urvu. Therefore, he explains the mitzvah that Rav Saadya counts (in the dibbur of לא תנאף) which sounds like kiddushin to be referring to pru urvu.
Rav Perla points out (Aseh #31, Chelek 1 page 176) that Rav Saadya counts milah in his Azharos under כבד.
Rav Perla didn't mention that the sciatic nerve wasn't included in the Azharos, and I didn't double check to confirm.