Steps one and three do not. These are incapable of occurring during any other step
shield: Casting Time: 1 reaction, which you take when you are hit by an attack or targeted by the magic missile spell
uncanny dodge: When an attacker that you can see hits you with an attack, you can use your reaction to halve the attack’s damage against you.
Also as an
instantaneous reaction a player casting
counterspell likewise is incapable of triggering at any point other than
while the GMis describing a creature casting it
Except there are a bunch of reaction abilities that can
only trigger outside of step2 because the conditions for their activation are
incapable of occurring during step2.
Shield: The hit is a potential hit, you do not advance to step 3 (description) until it is an actual hit or an actual miss. Once that's resolved proceed to step 3.
Uncanny Dodge: the damage is potential damage and you do not advance to step 3 until it is actual damage. The amount of potential damage is declared, the player declares uncanny dodge - actual damage is assessed. Proceed to step 3.
Counterspell: The DM declares the Baddie is casting a spell. Player declares counterspell, if Baddie is a caster he may ALSO then use his reaction. Each counterspell is resolved to see if it's successful (even if the 1st isn't, baddie still loses the spell slot). After resolution proceed to step 3.
Seems to me, the only problem is if the DM jumps to step 3 early:
DM: The Monster hits for 20 points of damage and nearly guts you!
Player: No he didn't.
DM: OK, guess I have to back up.
That's too early and both looks clunky and requires a weird retcon.
But if you just make sure to resolve all proper actions in step 2, that does not happen:
DM: The bugbear will hit you unless you can do something about it..
Player: I cast shield, my AC is 20 for this and the rest of the round.
DM: Ok then. The bugbears morningstar clangs off your shield, that was close!
or of course:
DM: The bugbear will hit you unless you can do something about it..
Player: I cast shield, my AC is 20 for this and the rest of the round.
DM: Ok then. You can't get the shield up quite in time - the bugbear's morningstar impacts for 11 HP of damage.
Step 3 does not occur until after the reaction is resolved. And in play it's fairly smooth, it's actually more clunky typed out than in play.