The FH side boosters should be almost equivalent to the F9 first stage (at least the Block 5 ones) - the difference will be mostly in mount points and the nosecone instead of the interstage. The center core is a bit more different with reinforcements to handle the forces. So from hardware standpoint the landings are the same thing. (One of the probable reasons for FH delay is to verify the landings before trying to land 3 boosters at once.)
The side boosters landing should be quite comparable to small-payload-to-LEO RTLS landing - the one done in December (Orbcomm). The booster has the same fuel capacity so the burn time will be similar and BECO will therefore probably happen somewhere around T+2:30 mark depending on booster throttling needed at MaxQ and the fuel reserves for the RTLS. But because of the heavy payload of throttled-down core the velocity and distance from the launch site will be probably lower so lower margins are needed for the boostback burn. In the end the landing should be quite similar to the one RTLS which was tried and successful.
The center core on the other hand will be going farther away and faster at MECO so possibly even with a boostback it will go through similar profile as current GEO/GTO launches which currently have 50% success from the 2 tries (SES-9 and JCAST-14).
So I would say that landing the side boosters has probability at least of the (currently 1/1) F9 RTLS and the core booster similar (slightly smaller than) the probability of GTO barge landings.
Edit now when FH exists and should launch soon:
For the center core - there were more succesful landings from GTO using 1-3-1 landing burns and there was another (most probably succesful - expendable booster survived) test of the 3-engine landing burn all the way down (the previous known test on SES-9 made a hole to the ASDS, lateer succesful GTO landings possibly used 3-1 or 1-3-1 landing burns? not sure if there was any other full 3-engines landing). So I would say the probability of the center core landing is quite high as SpaceX is testing and polishing these profiles. For the first launch even more so as the ASDS position seems to be quite a bit closer to the land than on standard GTO launches, suggesting somehow lofted trajectory for the interplanetary launch.
About the side boosters - for the test flight they got the titanium grid fins where the (expected to be faster and "hotter") center core has aluminum ones - a suggested reason being the nose cone being shorter and aerodynamically different than the interstage making the grid fins less effective. So the boosters will be a bit more different from normal F9 than I previously expected.
After the first FH launch:
Well, statistics and reality.. The side boosters are fine, the center core is not, somehow the igniter fluid reserve was not enough this time - so a new failure mode after all these tests. Currently not publicly known if bad valve, miscalculation or something FH-center-core specific.