All 3 of these weapons still lack a unique flair that really sets them apart as something special; That is, other than the fact that they all have great base stats, can swap elements on command, and have slightly different (but not more powerful) traits than are seen on standard weapons. I know that seems to contradict itself, but just because something like the legendary Khvostov has 3 choices for mag and barrel options, doesn't make it different in a way that is really meaningful (similarly, switching between elements on these weapons does nothing significant).
[spoiler]Borealis still has a hyper specific activation trigger for its exotic perk, which not only makes it impossible to utilize when there are no LIGHT shielded enemies, it also makes the weapon feel very unrewarding in general. Even if it were given innate Adaptive Munitions and could activate on any shield break (which seems like something Bungi would do) it still wouldn't be a viable option.
As for Dead Messenger, it's just a Triple Wave Frame GL, and Hard Light is just a normal auto with Double Ricochet Rounds[/spoiler]
Rather than simply giving these exotics a slightly better Collective Action, I think each one should be given a completely new, unique set of behaviors and effects that change based on the selected element (something beyond barebones Keyword nonsense). This would not only make each element on each weapon significant in some way, it would make each weapon stand out and stand apart from each other. That might sound confusing, so here's a hypothetical example of what I mean:
Dead Messenger:
Arc element, causing damage or landing kills spawns additional wave grenades
Solar element, shoots a ring of grenades that disperse waves outward on impact
Void element, double fires with much wider waves
Hard Light:
Arc element, causing damage increases fire rate, reload speed, range, and projectile tracking strength
Solar element, rapidly damaging targets causes expanding rings of fire to erupt around you
Void element, fires more slowly, projectiles stick into surfaces or targets, pull them inward, then detonate
Of course these are just some baseline examples: not ones I'm particularly fond of, but they get the point across
-
Going from guns that are meant to feature all 3 light elements to guns that feature 3 exotic guns in one seems slightly unrealistic but best of luck.