All 3 are similar in they stay on the Reef, move from site to site, and have daily transfer vessels come out and back.
Dive sites are also comparable, it’s really just the style of accommodation and vessel.
Ocean Quest is generally considered the best of the 3 in terms of accommodation and facilities (4 start I guess0 and its had a recent refit. Prices vary depending on what cabin type you want.
Reef Encounter is similar (its an SSI boat not PADI, which will make no difference unless you are looking to take courses etc).
Kangaroo Explorer is generally considered the budget option and you get one less dive (6 instead of 7) as the transfer vessels does not get out there until mid afternoon. You go via Fitzroy Island to get there.
With Reef Encounter your transfer vessel takes you direct to the boat and so all your dives are off of the overnight boat. Ocean Quest you have your first 2 dives off of Sea Quest (transfer vessel) and then transfer over after lunch.
To7 be honest I thing Reef Encounter and Ocean Quest are very similar internally and you cant go wrong with either of these 2.