Green is right-of-way boat for Red and Blue and therefore obstruction.
Because all three boats sail in the same direction Green is a continuing obstruction
and rule 18.5 applies.
At the moment, the overlap begins there is no room to pass between Green and
Blue and therefore Red is not entitled to inside room and shall keep clear.
Blue shall however keep clear of Red under rule
11. If there is a conflict between rule
18.5 and rule 11,
than rule 18.5 has precedence
(Preambel to Part C-rules).
See also case 16.