A bluejaw pair would be the safest, as well as a nice addition. At 3" is your bluejaw showing strong male colors yet, or does it look female. I've found the bluejaws need to over 4" to make a definite id on the sex. Many times I've seen people try for a pair and end up with 2 males as the apparent female matures more.
If you really had to have a niger, it would be challenge but doable. Nigers sadly are a dime a dozen and it's pretty easy to get one a couple inches. They usually aren't that aggressive, may or may not bother inverts. As far as living in a reef, I've seen it work with smaller ones. I haven't seen any large ones to know if they get that destructive nature as other triggers when they get over 5" or so. Two males may have a problem, niger males have streamers that usually don't show up until around 3- 3.5".