I`ve played in GC3 185 hours whith about 20 hours of multiplayer, and... there are some triubles whith trading:

1. In diplomacy there is variant to buy almost all(!) planets and\or starbases from any other faction for almost nothing. So anyone could dramatically expand his territory whithout any difficulties. I think that planets should not be tradeable, or at last they shoud be MUCH MORE expencive (about 1000 times to present situation). Starbases also should not be tradeable, except that ones, that placed on neutral, or buyer territory. 

2. Trade routes I think, shoud be:

a) breakeable as it was in GC2 (battleships should be able to intercept enemy traders and after several losses trade route will be closed).

b ) there should be transit tax for trade routes, that lie through other faction territories. Tax rate should be part of income proportional to lenght of transit part of route.

c) in accordance to "b" econonic starbases should increase income of all trade routes that lie through it`s influence zone.