I admire the teaching and lifestyle of St. Francis of Assisi. I wear a tau cross. I am planning to join a 3rd order in the near future. Is the wearing of the cross something that should be reserved for members of the order only or am I in any way disrespecting professed Franciscans?
No, it is not a sign of disrespect to wear the Tau Cross.
Anyone is free to wear the Tau Cross. As long as it is worn respectfully, as with any Cross, Medal or Crucifix.
You may want - since you are seeking that vocation - to hold off for a bit in wearing it …so that when you join it will have greater meaning for you. Just a thought not an obligation of course. We can all wear a Tau -tis a cross. I bought one in Rome (and I am not Franciscan) - but have not worn it. I prefer a medal.