Vegemamato, I second the suggestion to ask in I'm Pregnant and so on, but I'm one of the people who would answer you there anyway, so here goes...
Descriptions of placenta previa can get really confusing, because words like "complete" and "marginal" can be vague. Is this a "marginal" previa meaning the placenta is marginal to the cervix, or a marginal previa meaning the margin of the cervix overlies the placenta? (When I was discussing previa with doctors, I finally gave up on words and switched to using my hands to illustrate - a fist for the cervix, and the other hand for the placenta. It was much less confusing.) *Usually*, a marginal previa means that the placenta is close to the cervix, but not over it. Double check that this was what the radiologist meant.
Anterior presentation in a previa case can be a complication for surgical delivery, but I'm going to encourage you not to worry about that too much right now, for the following reasons:
1. YES, most cases of placenta previa diagnosed at this stage clear up well before delivery.
2. NO, the placenta cannot move down further over the cervix. (As your uterus expands, all the points within your uterus - like the placental implantation site and your cervix - move away from each other. Imagine if you drew two dots on a balloon and then blew it up - the dots would get further apart as the balloon expanded.)
The course of treatment that the radiologist described to you is quite drastic, and, IMO, pretty unlikely. It wouldn't be appropriate for the radiologist to put you on pelvic rest or anything like that, but I cannot *imagine* having to wait three weeks to talk to my OB about this issue. Call on Monday. Ask if your appointment can be moved up. Ask these questions in detail, in person, as soon as you can.
I have been there with previa, and done that, and my advice is:
If you have red panties, put them away for the duration.
Get a follow-up ultrasound on the best imaging equipment available to you (because placentas don't move toward cervixes, but crappy imaging equipment may produce inaccurate diagnoses).
Watch your hydration, pee often, and generally keep an eye on anything that might cause contractions.
Remember that, most of the time, this problem goes away by itself.
Unless you start to bleed (in which case, get to the hospital immediately), get a follow-up in ten weeks or so and don't worry until then.