I think that your scenario applies even more so for the Chargers than it does for Dallas. I have repeatedly heard that the Chargers are not tied into any one player and are open to moving down.
A team looking to trade up for a quarterback could and would do it there rather than risking the chance of Dallas having the quarterback on their board that they loved and being unwilling to move.
I think that we as Cowboys fans would like for pick #4 to be a windfall, but the #3 pick appears to be significantly more valuable and likely in play too.
Yes, I agree with your general thought process regarding the Chargers and the #3 overall pick. While the Chargers have the same opportunity to trade back as we do here are the impediments to the CHARGERS doing that:
1. The #3 pick is worth even more than the #4 pick is, and while it is just one draft spot, the Chargers will want full value for the pick. I'm not saying that the Rams won't pay whatever the Chargers are asking, I am just saying that for every spot you move up at the top of the draft the more prohibitive the cost. The dropoff in talent from the #3 pick to #15 pick is going to be vast.
The top 3 will probably go something like this on most boards:
Tunsil
Wentz
Ramsey
Either way, neither Tunsil nor Ramsey will likely be on the board for the 4th pick unless both QBs go in the top 3 but it is ALMOST CERTAIN that at least one will of the two be on the board at #3 overall.
2. It is rumored that the Chargers REALLY like Ramsey and he should be on the board when they pick at #3 overall. So they will probably have to decide if they want the concensus best defensive player the draft in Ramsey or a much lesser talent in a trade down. If the Chargers take Ramsey, then it is arguable that if you don't want to take a QB there probably isn't anyone the Cowboys would really want at #4. If the Cowboys go on the clock and one of the top two QBs are one the board, then Ramsey is probably long gone.