::) 3/4 throttle normally means if you did not change your config file:
a.  You are dragging your rear stair door and the APU cart behind you
b.  You added an extra 0 in your weight total somehow.
c.  The parking brake is on.
d.  Finally, my favorite, if you find that you are having to add way too much power just to taxi - See if your landing gear is up!

Seriously, barring any of the above I cannot think of any reason for your power to have to be so high to taxi.