I have an extra 3-wood shaft (TaylorMade REAX Superfast 50g R-flex) that I want to install in a 5-wood.  How much should I tip trim the 3-wood shaft for the 5-wood installation to maintain a standard R-flex?

You should also take into account the weight of the 5W head, relative to the 3W head you took it from. There is no standard for headweights, and that appears to be a very light shaft. I imagine its been designed for a light head with longer shaft.