If your budget is tight, why are you looking for such a specific kind of monitor? You might do better just looking for the best overall monitor in your price range.
Well, I want to have a dedicated mid-range speaker. Since I don't have alot of money to spend, why would I want to waste it on something I won't be happy with?
Well, you're not likely to find an active pair of 3-way speakers in the $400 range, but you could maybe look at something like 2-way's with a sub. Again, in the $400 range, you're not going to find much, but Samson makes the 2-way Resolv 50a's and the Resolv 120a sub that will cost about $400 for all three. Never heard the Samson speakers, so I cannot account for their quality, so you'd have to check them out yourself.
Personally I think a good set of 2-ways would gain you better quality overall. Lot's to choose from the $400 range.