A very easy species to sex visually.

Cocks are much pinker overall than the hens (cock left)

Even more obvious in side profile (cock left)

And finally, in typical estrildan style the birds can be sexed in the ventral region - Cocks show much more red colouring.

So just to recap the last 3 species we covered, they are very similar so I will point out the key differences between them :

Left - Sundevall's Waxbill (more fawn coloured than the other 2) Middle - Red-eared Waxbill (pinkish wash overall) and Right - Common Waxbill (finely barred allover)

When comparing heads the Sundevall's is quite different with a redish-black beak. The other 2 are very similar.

But the easiest way to identify the different species is by looking at the rump and upper tail coverts: Left is the red rump of the Sundevall's (now you can see why it's also called the Rosy-rumped Waxbill) Middle is the black rump of the Red-eared Waxbill (and you can see why it's also called the Black rumped Waxbill) Right is the Common Waxbill with the barring uniform from head to tail.