Hi, guppies aren't that hard to sex, provided that they aren't fry..
The dead giveaway for the females are the presence of the
gravid spot.
This is a dark/black spot at the bottom end of the fish (by the anal fin), these are due to fry/eggs present in the female, males do not show this.
As for the males, they have a
gonopodium which is a sort of extended anal fin which is used to fertilize the female.
Also, if you still aren't sure, Males exhibit more color and somewhat larger/longer finnage that females (females mostly are dull colored).
But its always very easy to tell by the gravid spot.
If you have any pics post them and we'll be able to easily tell..
As for the fish, if the LFS was true and you got 3 females 1 male, then assuming a female died, you now have 1 male 2 females.. do your best to keep both females alive, as if there was 1 male and 1 female, the male would constantly harass her, whereas having two ensures that his attention is divided between them.
So long as the male did not die, and you have at least 1 male and female, be ready to be grandpa
They
will make fry.