Saad - just below average, about the 45% percentile. That being said he improved significantly this year (~20%) and the PAV system doesn’t necessarily capture everything about a player like Saad (metres gained aren’t used, bounces etc). I wouldn’t pay more than ~pick 30 though based on this.
Smith - When he was up and about he was on average in the best 25% of the players in the league (2014-2016). Dropped to a touch below average this year and that should hurt his trade currency. He is worth more than Saad based on his already shown ceiling, but I would say early to mid 20s.
Stringer - very similar trajectory to Smith, except at his best he was getting close to elite (note that this was 2015) and he has dropped steadily now to a PAV of about 14 which is still well above average but a lot less than his peak of ~18 (top 10%). Worth a touch more than Smith based on this, maybe late teens pick.
Both Bulldogs and GWS are kidding if they think they can get maximum currency for players who have taken two consecutive steps in the wrong direction (and this is purely statistically, without the external concerns, although causation and correlation). Out of the three, Saad is the only one going in the right direction, so it will be interesting to see how it all plays out.
For reference, based purely on this year, Stringer would be our 10th most valuable player, Devon Smith 13th and Saad 14th. This would push Ambrose, Green and James Kelly out of the top 22 on rankings, so our net PAV would have improved by about 7 points, which is significant but not huge. Those three players either play at their best (smith, stringer) or keep improving (saad), and there is significant potential for increasing the total value of the team next year however.