Disney does charge extra, IIRC $25 pp for each adult after the first two. I would compare prices on Disney's website: 2 adults vs. 3 adults to see how much extra they truly charge. Then do the same search on Orbitz and see if the 3rd adult surcharge is equal or not. If Orbitz charges more, I would wait and add them on at the time of the chat if possible. I'm not really sure if the agents are able to make price changes too. I guess it can't hurt to ask. JMHO, but I think if the difference in prices was like $5 between 3 adults on Disney vs. 3 adults on Orbitz, I probably would let it go and just focus on adding any children. I have had Orbitz chat agents tell me I need to modify and add the child but I simply explain that Orbitz charges me extra then when I know for a fact that Disney does NOT. They make a feeble attempt to say that all hotels charge for >2 guests but I have found that to be untrue except hotels on the strip in Vegas and who really needs to bring kids there. lol.
As for adding an infant, I believe they still must be on the reservation for legal occupancy purposes but the big difference there is that an infant doesn't need a ticket. So unless you're buying tickets from Orbitz, it's a moot point. Simply add as another child and there shouldn't be any extra charge for doing so. I don't recall if Orbitz lets you make requests (we rarely have any) but I would try to request a crib.
Keep in mind that booking through Orbitz means they are now your
travel agent. Anything you change must be through them. Since Disney pays a commission to all TAs, they generally do not want to:
1. step on a TAs toes so to speak or
2. perform the duties that a TA is being paid to do.
So calling Disney directly will usually leave you disappointed. Just something to keep in mind. This can be very frustrating when people book flights through 3rd party sites such as Orbitz, Expedia, Travelocity, etc. If you're at MCO and find that your flight home was cancelled due to bad weather back home, instead of having the airline help you, the travel agent has to help. At least this is what I have read anyway. That would drive me bonkers.
I do know with
DCL, if a TA is involved, everything goes through the TA. I don't even try to contact DCL directly.
If Orbitz has no way of letting you request a crib, then I would contact Disney probably a week in advance and make sure it's noted on your reservation.
hth