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.
Source: Obsidian Flames FAQ; TPCi Rules Team (2023-08-10)

Relevant cards

Note: All applicable cards may not be listed below.
SV3_52
SV3_52