Those are 3 great islands. Corfu is much more green than Crete and Santorini and does have a different feel (more Italian but it’s no where near Turkey). The trouble with doing all 3 is that Corfu is on the opposite side of Greece from Crete and Santorini so you’d need to fly via Athens. It’s better for most people to visit another Cycladic island (Naxos, Paros, Milos, Folegandros) instead of splitting up their trip between the two sides of the country.
Ferries are not really cheap any more – certainly not like they used to be – but the vessel fleet is better, safer and faster than in previous years so the cost is justified. You can get exceptionally cheap deals on longer-haul routes if you are prepared to forego a booked seat: €14 v €40 on a run to a mid-distant island, but it’s probably wise to consider taking fast catamarans to get to core islands (Mykonos, Santorini, Paros etc.) and even then, go the extra 10/15% for Business or even VIP class for the extra comfort. On longer hauls it can be cheaper to fly if you seek out flights online and book beforehand.
Budgeting for Greece is always going to be subject to what your expectations and needs are. Assuming that you, like many travellers, like to eat a filling breakfast, savour maybe a light lunch and feast on a heavier dinner (or vice versa), enjoy a drink with your meals and are not totally vegetarian/vegan and prefer to sleep in comfort and cleanliness, then there is a set of figures that can be guesstimated.
Can you see all the major sights in Athens in one day? No. But you can see the Acropolis, Acropolis Museum, and the top historical sites of the Plaka in one day. If you had an extra half-day then visit the Archaeological Museum in Exarcheia. That still leaves many great sights but you will have seen all of the iconic Athens attractions. Adding Naxos is always a good idea. You might even enjoy it more than Mykonos (but no nightlife like Mykonos).
We are planning our honeymoon in Greece. We can probably take up to two weeks. One of the places we want to go is Santorini. We would like to visit one or maybe two other places if possible. The other places we were looking at are Athens, Mykonos & Crete. Which of these would you recommend with Santorini if we were thinking of visiting 2 or 3 places total?