Actually, you can't "rune it down" to 3 Ticks. Please read the text of the Celerity Rune:
Forged in Magic, Volume 1, page 7 wrote:
Celerity
Description: This Rune allows you to move or attack
with incredible speed.
Wand: No effect.
Weapon: When a weapon with this Rune is used to
perform a Combat Maneuver you reduce the Recovery,
Speed or both (to a minimum of 3 Speed and 1 Recovery).
Note: If the weapon’s speed would drop below 3 add +1
damage instead
Please note it says COMBAT MANEUVER.
Arcanis The Roleplaying Game, page 310 wrote:
Combat Maneuvers (which consist of Weapon Tricks or Martial Maneuvers)
This means that you can only ever get your Gladius to Speed 3 by using a combat maneuver which has a speed of +0 (X), or with the Rapid Strike Weapon Trick.
You are correct that Guns and Crossbows cannot fire any faster than 2 ticks in the current rules set, but Exceptional provides additional Rune Slots compared to Fine or Good Quality which is nothing to sneeze at. In terms of Bows, Exceptional bows are no less powerful than any other weapon in terms of speed vs. damage. They do have a 2 tick reload, but there are Talents and Martial Techniques which ameliorate that rather short reload.
-Ah, that was not clear to me at first read, that the speed referred to the speed of the maneuver and not the weapon speed. In any case, the distinction is partially moot as most melee attacks will be using some kind of maneuver unless they are in recovery, so the benefit of speed reduction still applies, although the math changes depending on the maneuver.