Assuming that the big lump on the underside of the fish is genitalia and not poo, female. But then again why whould it's genitals be so swollen?
If it's poo (likely), then you'll have to perv up your fish yourself.
One couldn't very well judge by head shape since the fish is only 3" and not fully developed.
Also assuming that this is easier to remember than what's in the link (been there, done it, forgot whet the hell I was looking at and what for...)
Male, anus and papilla should be of similar size.
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Female, papilla is larger than anus.
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