The Xperia Z3 now sports a narrow border design ,more compact with bigger display, thus,comfortable for one handed use. While the Z1 had a 5-inch screen, it felt bulky as compared to Xperia Z3 which has a bigger 5.2-inch display.
We can distinguish between the three smartphones as:-
Xperia Z1:5-inch display, 144 x 74 x 8.5mm, 171 grams
Xperia Z2: 5.2-inch display, 147 x 73 x 8.2mm, 163 grams
Xperia Z3:5.2-inch display, 146 x 72 x 7.3mm, 153 grams
Xperia Z3 has a far better camera than the earlier Xperia Z2 or Xperia Z1 models, especially the 4K video recorder.