The power of Rising voltage is doubled in electric terrain, and electric terrain gives a 1.5x boost to electric type moves. Do these boosts stack to make it a 3x move or is the electric terrain boost ignored?
It does get both boosts. Though it's a 1.3 boost not 1.5 it was nerfed.
I tested it in the showdown calculator.
70 base bower move from a blank silvally to a blank sillvally is 15.4 - 18.1%
140 base bower move from a BS to a BS is 30.5 - 35.9%
Now make them electric and add terrain you get
rising voltage 39.2 - 46.5%
70 bp electric move in terrain 19.9 - 23.5%
140 BP electric move 39.2 - 46.5%
Essentially if it was just the base power boost you'd get the 30.5 - 35.9% but since you get both because their are 2 separate criteria to be met. The user must be grounded to receive terrain boost and the receiver must be grounded for rising voltage to get double bp in the terrain.
These values aren't including stab so the damage value with is even higher
No, Rising Voltage only gets the 2x boost from its description to become 140 and not the additional boost from Electric Terrain. If course, this only applies if you are affected by Electric Terrain as well.
Also, I tested it On Showdown!