If you still have the 3G, connect it to the computer, go into Info, setup calendars to sync with Outlook or any other program that may be listed there if you prefer it, then sync.
Then, connect the 3GS, do the same thing, and sync.
These may not be the
exact directions, but with this guidance you should definitely be able to figure it out.
(If you do not still have the 3G and it was never enabled to sync with a program, then unfortunately your calendar events are lost)