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

Echoed VoiceDuring your next turn, this Pokémon's Echoed Voice attack does 50 more damage (before applying Weakness and Resistance).



Drain PunchHeal 20 damage from this Pokémon.
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.