There are most certainly flights between Athens and Rhodes and Athens and Crete all year round. There are most certainly ferries between Piraeus (Athens) and both Rhodes and Crete all year round too.
For these routes, you're simply looking too early for the details to have been loaded. Airlines are Aegean/Olympic and Ryanair.
What is more problematic is the link between Rhodes and Crete. There will be flights and there will be ferries, but the ferries are likely to be just two a week, and the planes may not be daily. SkyExpress operate the flights, which aren't loaded yet.