提案/Suggestion
Shelling > Shelling > Wyrmstake Cannon is a very strong combo for gunlance, however when using Shelling after a perfect evade it doesn’t count for the combo so you need to execute 3 Shelling attacks before being able to use Wyrmstake Cannon. At best, this is an annoyance that delays the combo, at worst this blocks the usage of Wyrmstake Cannon on wide gunlances because they will instead execute Shelling twice and then be forced to reload since their base shell capacity is 2. Also the same thing happens after blocking an attack.
My suggestion is to consider Shelling attacks done after a perfect evade or block as part of the Wyrmstake Cannon combo, just like regular Shelling attacks do.
現状/Current Status
In order to be able to use Wyrmstake Cannon after a perfect evade (or block) you have to use Shelling 3 times in a row. Normally you only need to use Shelling twice before Wyrmstake Cannon.