Answer:
violates ethical, but not legal, standards.
Explanation:
Sherman act was created to prohibit restrains on trade of any collusion by different parties to form a monopoly or control price.
The act does not however prohibit all restraints of trade, bit rather those that are very unreasonable and harmful to competition.
In the given scenario the three companies only agreed to bid lowest for the 3 project under consideration.
Their action does not give them unfair advantage over other firms and may even lead to a loss on their part.
They do not have a strategy that will guarantee an edge over other firms.
So this is an ethical violation but not a legal one.