Parlay EV Question

I know that parlays are an -EV bet except for extreme circumstances. For instance, a 3 team parlay paying 6:1, assuming the game lines are correct, is a -EV bet. How would the EV be calculated?

Edit:

Is it something like this?

If spread is accurate, then odds at -100 are:
0.5*0.5*0.5 = 0.125 = 7:1

but pays only 6:1 thus you lose the difference.

Or is it even worse, like this with bets at -110:

(0.5*10/11)^3 = 0.0939 = 9.6:1

So the only time a parlay is a good bet is if it pays 7:1 (or 9.6) or better?