If I use Meloetta ex's "Echoed Voice" attack three turns in a row, would I do 190 damage on the third turn?
No, the bonus damage doesn't keep stacking. It is only applied for each next turn, so you would do 30, 110, 110, etc.
Echoed Voice



x2
PsychicDoes 20 more damage for each Energy attached to the Defending Pokémon.


Echoed VoiceDuring your next turn, this Pokémon's Echoed Voice attack does 50 more damage (before applying Weakness and Resistance).
If I use Meloetta ex's "Echoed Voice" attack three turns in a row, would I do 190 damage on the third turn?
No, the bonus damage doesn't keep stacking. It is only applied for each next turn, so you would do 30, 110, 110, etc.
If I use Seismitoad's "Echoed Voice" attack three turns in a row, would I do 320 damage on the third turn?
No, the bonus damage doesn't keep stacking. It is only applied for each next turn, so you would do 120, 220, 220, etc.