You can't tell on chicks unless you know how to vent sex, which is an uncommon skill and not very easy to learn. Though there is a 2-5% margin of error, if you purchased three pullets and got three cockerels then the seller was either mistaken or lying.
Once the birds get older, there are a variety of ways to tell. Males will have larger combs which will begin to redden up anywhere between 2-10 weeks of age, whereas pullets will have small and very pale pink or yellow combs until they near laying age.
At about 10-13 weeks cockerels will develop male specific saddle and wing bow feathers. They will be long, sharp, and somewhat see-through, especially around the edges.
Cockerels will (often) stand taller than pullets.
Cockerels will (often) have larger feet than pullets.
Cockerels will (often) be bolder and friendlier than pullets.
(These last three aren't necessarily all that accurate on their own, but used in combination with each other and comb/feathers they can be quite helpful).