The males have a much steeper profile and fewer dark patches (when in normal coloration.
Females more torpedo shaped profile (this can be subtle) and have more dark markings.
At 3" these may not be apparent as the previous poster noted.
Here is a 8-10" male.
now a female, note the different shaped profile, and few more dark markings.
Alpha males can be very hard on females, especially if your tank is under 300 gallons.
My suggestion would be to get 6-8 juvies (more is better), and allow them to grow up together in the largest tank you can get, and expect the alpha male to kill all other males, and then kill off all unreceptive females.
If you are lucky, you'll end up with a bonded pair. (I believe all the killing helps bond the pair)
The 1st time I had them, I started with 11 juvies, but a a small tank of 150 gallons, the alpha killed all 10 others (almost over night when he hit 5").
The 2nd group, I ended up with a pair, that spawned( below) male front).
My first group came from Don Conkel, and he had a copasetic trio, living together in a (what appeared to be) a 500 gallon vat.
I believe in any tank under 300 gallons, trying to keep more than one argentea is an exercise in futility 80-90% of the time, once the alpha hits 5" of so.