I was going to write up a paragraph explaining why the waiting time can be used, but decided to just math it out and see for myself what the difference in EV was.
So you can compute the EV of PC by adding together the following:
The probability of drawing PC in your opening hand (value of PC for the remaining draws - 5 - 1)
The probability of not having drawn PC by draw X drawing PC at the Xth draw * (value of PC for the remaining draws - 5 - 1)
And you would sum this over all draw points where installing and using PC has positive value (i.e. you will draw enough over the remaining game to make up for the install). Also it's - 5 - 1 for the 5-cost install and the click to install.
The one variable here is the expected number of draws you plan to make throughout the game. If you can win the game with your opening hand, PC is useless. If you're going to draw through your whole deck PC gains value. So we have to do a little sensitivity analysis to see what kind of values we get.
What I found is (drumroll, please) I'm lazy and could've done a better job. I used the expected waiting time because this would be approximately correct in a world where the probability of having drawn PC as you go through your deck increases linearly. This felt sufficient, and I figured it'd be pretty close, but apparently I was a bit off.
Here's what I found:
If you expect to draw down to 5 cards left, the EV of the first copy of PC you draw is 24.6 credits with 3 copies, vs 20.5 credits with 2 copies.
If you expect to draw down to 10 cards left, the EV of the first copy of PC you draw is 19.9 credits with 3 copies, vs 16.4 with 2 copies.
If you expect to draw down to 20 cards left, the EV of the first copy of PC you draw is 10.7 credits with 3 copies, vs 8.1 with 2 copies.
So yeah, that would've been a much more complete article if I had done it this way. The approximation I used overestimates the value a bit -- I think most games I've played with this deck take me down to about 15 cards remaining, so a third PC would net you about 3 credits in hand. That's the equivalent of replacing it with a Sure Gamble (you're netting +4 but taking a click to play it). I guess in the end, same same.