Since WC3 takes place three years after WC2, I do have a theory that the uniforms from WC2 were still in use but only in a more formal area such as planetside or space station duty. The uniforms seen in WC3 were more rugged looking so they could be utility uniforms to give personnel a bit more comfort especially after the Battle of Terra. Tolwyn wears a more formal uniform and I wouldn't put it past him to find the "utility" uniforms ugly and preferred to keep up appearances. If he had it his way, he would have all bridge officers on the Victory and all O-5 and above wear the more formal uniforms from WC2 unless they worked in Engineering spaces. He would let O-4 and below or personnel he doesn't deal with daily slide. As long as they're clean of course. The Treaty of Torgo signing would have everyone in full dress uniform. Production and limitations when shooting however, yeah everyone just wore the same uniform. Not to mention having to change the rank insignias that resembled US Military insignias from WC1 and 2 to something more original.