Some agender people reject the NB and trans label, others use it. Personally, I've used all three but there was a point where I specifically identified as a gender but didn't think trans or NB were useful labels, so I rejected them even though I thought I'd technically fall under the broad definition.

For me, I was attracted to the agender label first because my basically feeling about gender was in was too autistic for gender. Before adopting the agender label, I was confused how someone who was autistic could be cis or trans. I thought gendered restrictions and norms were annoying and felt like I tended to ignore gender more than most on accident (ie: someone else pointing out that I was the only guy in a group on a couple occasions... Once when it was supposed to be girls' turn to change on the bus... Oops). Agender people at least on reddit seem to be overwhelmingly ace.

NB is a big umbrella. It would include agender, bigender, poly gender, demigender, gender fluid, etc.