Need to trim inches quickly

I'm not going to say it's impossible to lose 3 inches in three weeks, but the odds are definitely not in your favor. As everyone else suggests I recommend looking into low-carb dieting as it'll probably give you your best shot at losing as much as possible by then. The only things to keep in mind really are that everybody is different; some people (like me) can lose weight very quickly on a low carb diet. On the other hand I know of some people who can't lose weight on a low-carb diet no matter how hard they try.

The other thing to keep in mind is that on an average sized person, 10lbs of fat = 1 inch around the waistline. It varies from person to person obviously but on average that measurement holds true. So if you want to lose 3 inches you're looking at about 30lbs you need to lose in 21 days. I'm not going to say it's impossible (anything is if you put your mind to it). It's just that the odds are not really in your favor. The amount of strain you'd be putting your body under to sustain losing almost 1.5lbs a day wouldn't be healthy either.

You may want to consider maybe just picking up a new dress though. Although you could take the dress you bought to a seamstress, chances are that they're not going to be able to let it out three inches. Depending on the number of panels and how the individual panels are cut and sewn together there's probably only room for an inch or two at best. Depending on the material the dress is made out of itself; sometimes if you let out a garment the material will retain the stitch marks too. On cottons, wools, and other non-delicate fabrics you can usually get the marks out by ironing the heck out of it with an iron. On delicate fabrics? As they say in Brooklyn: fuhgedaboutit!

You may find that if the seamstress is a miracle worker and can do the work (and I'm talking about an experienced seamstress that knows their craft), you may find that it'd be less expensive just to buy a new dress.

Besides the crash diet, letting the dress out (or outright buying another one), there is another option here that everyone is glossing over...

You may want to talk to your husband and see if you both can postpone the ceremony for another month (giving you seven weeks to lose the weight). Granted it won't be exactly on your ten year anniversary, but think of it this way: You've been together for 10 years now, what's another month? Besides; he'll make out good in this regard since you'll be less stressed out the days leading up to the event too (like the saying goes: If the wife isn't happy, the husband isn't either. ).