What does Phase 3A mean for avionics?
It is just a matter of technology, it turned out that phase 3 are subject to interferences from wireless devises, resulting in disappearance of displayed data. 

We have been alerted of this some years ago and as long as several similar equipments are simultaneously  functional, it is probabilistically nearly impossible to loose the parameters on all these equipments simultaneously.

I found something about this on the following website, but may be on Honeywell website you will get more details

https://www.ainonline.com/aviation-news/2014-10-01/faa-orders-replacement-737-and-777-display-units

So the  trouble of interferences is specific to phase 3 DU, phase 3A corrects this effect.