I've no personal experience of retained placentas, unfortuantely.
Did you try for a physiological 3rd stage on either of your previous births?
If it were me, though, I'd be asking why they needed to get an IV line in when you're in labour "in case" they need to give you anything afterwards. I'm not trying to trivialise it (and I may well be wrong) but I didn't think a retained placenta was usually a "have to get you into theatre ASAP - everyone grab something and run" style emergency unless there were other complications. In which case they'd have time to site the drip. What do they want to be able to put down it, anyway? I thought synto for a managed 3rd stage was usually intra-muscular? Why do they even need a drip? Surely as long as they've got the canula in attaching the drip would take seconds? If I'm right on all of that then I'd be refusing to allow them to put a drip in on those grounds, and probably refusing to allow a canula as well as at best its going to be annoying. (Not that you need to tell them why, but it might make life easier!)
I'd expect them to have your maternity notes from the previous hospital. If not, I'd be asking for another appointment once they've accessed them. Understanding what was done during your previous two births would seem fairly important to planning this one, and trying to do that with out the notes strikes me as a trifle futile.
If the consultant is adamant its important to get a drip into you before the birth, ask him what research he's basing that on. Make notes. I'd be asking very pointed questions about how restricting my movement and options during the 1st and 2nd stage is likely to reduce the risk of a retained placenta. At the end of the day, they can make recommendations but its your body, your baby, your birth and your decision as to what actually happens. In the words of Mary Cronk, "'Allow' is not a word to use to a mentally competent adult".
(can you tell I'm a nightmare patient yet )