I liked R3's bleepy vocals. Besides the obvious external differences between astromechs, I've also enjoyed the few subtle differences that the universe has explored over the years. There aren't many, so when we get to experience them, it is a treat.
Being a more developed character, R2-D2 is of course the most familiar of all the astromechs. But I know some R5's have distinct sounds, and Obi-Wan's R4 in AOTC sounded unique as well. So this was a nice change of pace. I suppose it was especially necessary from a storyline perspective in this instance, since Anakin had to spurred on by R2 beeps off in the distance while still in the presence of this "R3." Like cheese or breadcrumbs to a mouse, this distinction was important for the plot to go on.