Suppose soldiers do commandeer Bob's house at the express direction of the President without hisBob's consent or compensation or following any procedure that amounts to due process at the express direction of the President. What does Bob do?
A federal trial judge hears the case and decides if the law is constitutional or not, and if it is held to be unconstitutional may decide that you areBob is entitled to a remedy.
Are the soldiers are immune to suit for money damages against them personally, which depends upon how clear it was to the soldier that he was acting unconstitutionally?
If the trial judge finds that the law is unconstitutional, the trial judge can issue an order saying so and that is the law of land that binds the parties If the trial judge finds that the law is unconstitutional, the trial judge can issue an order saying so and that is the law of land that binds the parties (including the U.S. in any other case presenting the same issue under a principle called collateral estoppel) unless someone appeals the case.
If the U.S. Supreme Court does decide to take the case, it can affirm that U.S. Court of Appeals ruling (which is then binding on all U.S. CourtsCourts as precedent), or it can reverse the U.S. Court of Appeals.
Only a handful of cases in the history of the United States have ever squarely addressed whether a law violates the 3rd Amendment so there isn't a lot of directly applicable precedent, but the judges would also consider how similar provisions of the constitution, like the 5th Amendment, have been treated and would consider law review articles and historical records about the intent of the Third Amendment as well.
But, it really doesn't matter if the violation is blatant or if it is subtle. The court even routinely rules that laws are unconstitutional not because they actualactually violate a provision of the constitution directly, but because they merely "burden" the exercise of a constitutional right. A law that effectively nullifies a constitutional provision would usually be invalidated. Sometimes lawyers informally and in private call an argument that is technically valid (for example, by creating a procedure albeit a meaningless one), "too cute." Arguments like that usually lose.
The U.S. Supreme Court routinely invalidates laws that violate the constitution only in very subtle ways (e.g., requirements that have been interpreted to pose minor barriers to voting could be held unconstitutional), and the U.S. Supreme Court now and then refuses to invalidate laws that seemingly blatantly violate the constitution (e.g., "In God We Trust" on U.S. coins).
Often, constitutional cases are resolved on the question of standing, or whether the right person has been sued, or by interpreting a law in a manner that is unnatural, in order to avoid having to reachaddress the question of the constitutionality of the law itself.