In Rashi's understanding of the Mishnah, the wives split their deceased husband's estate according to their claims: each piece is split between the wives claiming it.
As the claims are $100, $200, and $300, this only works until his estate is $300. What if his estate is worth $400-$600? If the highest claim is $300, nobody is claiming that money, yet he has enough to pay each wife more than they've gotten. It doesn't seem right to withhold money from them if he can repay his debt. But the same question arises: who gets first dibs, and how much can they claim?
Doniel Filreis
1) If the estate is worth $400, this means that the wife who has a claim to $300 also has no claim to the fourth hundred, in the same way that we say that the wife who claims $100 has no claim to the second hundred. Therefore, if the estate is worth $301 up to a million dollars, the Halachah is the same as when it is worth $300.
2) Your argument that it is not right to withold money from them if he can repay the debt is in fact the argument of Rebbi (near the bottom of the page) who disagrees with Rebbi Nosan. Rashi (last line of 93a) writes that, according to Rebbi, all the husband's property is liable for payments to his wives' Kesuvos.
According to this, if the estate is worth $600, all three wives will receive their full Kesuvah.
The Halacha follows Rebbi (see Shulchan Aruch, Even ha'Ezer 96:18).
Kol Tuv,
Dovid Bloom
(Have you seen the Shi'ur on this Sugya that Nobel Laureate, Professor Aumann, gave to our Kollel, available at "Lectures" on dafyomi.co.il?)