From long (and occasional unhappy) experience:
Three has the potential to be mistaken for "two" (and vice versa).
"Tree" is correct, sounds somewhat dorky, and IMO the most important annunciation to prevent confusion is the "ee" sound. Likewise with "two" the "oo" sound should be clear.
Not that I'm actually suggesting a personal variance to the standard; what I'm suggesting is to learn why the standard is the way it is. Understand what is behind the rules, and you understand why they should be followed and where they might be not quite adequate, in rare cases.
Five and nine are regularly confused if not pronounced correctly. The ICAO pronunciation should always be used with these two.