first of all, you haven't indicated how the house was acquired and for what cost ...
you are not taxed on the proceeds of the sale but rather on the amount your proceeds (less sales expenses e.g. commissions) exceed your "basis" in the property ... your "basis" is the original amount paid (if any) plus improvements made to the property ... so, if you sold the property for $99K (and paid $9K in commissions) your net proceeds would be $90K ... but let's also say that you paid $50K to buy the house and added another $10K in repairs ... your "adjusted basis" before the sale would be $60K ... your taxable gain would be your net proceeds, $90K, less your "adjusted basis", $60K, giving you a total taxable gain of $30K ...
I must admit that i haven't done taxwork for about a billion years, but i don't see a way to pay the taxes as part of the sale ... i think you have to distribute, pro rata, the gain to the 3 owners ... assuming you share equally in the property, each owner would have to report a gain on the sale of real estate of 1/3 of the taxable gain ... if the property was a primary residence, each owner may be able to defer the tax due on the gain under certain conditions ...
finally, i'm not sure that the way you're wording the agreement makes any sense ... i don't think you can say that each owner is responsible for 1/3 of the taxes due because the tax due is conditional based on each owner's individual tax situation ... the only way that might not be true is if the house were owned as part of a partnership agreement or perhaps as part of a trust ... but that's beyond stuff I know much about ...
you can download
IRS publication 523 by clicking right here ... the publication describes all the rules about sale of residence ... it might be worth checking with a tax account ... it might also be important, if you have control over the sale date, to determine whether you're better off deferring the sale until after the 1st of the new year ... then, the tax issues wouldn't affect your return until April of 2006 instead of 2005 ...
hope this helps ...
- wt2