Challenge 1: Three Variables Summing to 9 via CLP(FD) — Possible Solution
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sum_demo.pl:

:- use_module(library(clpfd)).

Query:

    ?- X in 1..5, Y in 1..5, Z in 1..5, X + Y + Z #= 9, label([X, Y, Z]).
    X = 1, Y = 3, Z = 5 ;
    X = 1, Y = 4, Z = 4 ;
    X = 1, Y = 5, Z = 3 ;
    X = 2, Y = 2, Z = 5 ;
    X = 2, Y = 3, Z = 4 ;
    X = 2, Y = 4, Z = 3 ;
    X = 2, Y = 5, Z = 2 ;
    X = 3, Y = 1, Z = 5 ;
    ... (every combination of three values 1-5 summing to 9)

Explanation:
Each of X, Y, Z is declared as a domain variable ranging over 1..5,
none of them assigned a concrete value yet. Posting X + Y + Z #= 9
immediately propagates that constraint across all three domains
simultaneously -- for instance, no variable could ever actually be
labeled 1 while the other two are both also forced below 4, since
their combined maximum wouldn't reach 9, so CLP(FD) narrows the
effectively-reachable combinations before label/1 even starts
searching. label([X, Y, Z]) then backtracks through every concrete
assignment that satisfies both the domain bounds and the sum
constraint together, offering each one via ;, in the same pattern the
chapter's own two-variable example demonstrated.

WHY THIS WORKS AS AN ANSWER
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This extends the chapter's own two-variable sum pattern to three
variables and a different target sum, confirming the same in/#=/label
workflow scales cleanly to more variables without any change in
technique.