Yup the coloration on picture one really screams male. Seems like there are a few distinguishing characteristics on sexing them and most of the males seem more colorful even as juveniles than the females. That thing has great color! Hopefully you are able to get a nice pair out of the trio and offer some nice colorful fish up for sale.
Have to disagree on that. You can see the trailers develope later on some fish. There's more than one factor for sexing rivulatus and the fin shape isn't 100% definative. Look at the most recent pic it's clearly looking male a few months later.