I would not worry about the proportional value. Instead, I would consider that you sold a strip of land for exactly the cost basis, you have no taxable gain, and your cost basis for the future is reduced.
For example, suppose your father's cost basis was $100,000, and he "sold" (was taken) a strip of land for $10,000 that was divided 3 ways. Each recipient would report that the cost basis of the strip of land was $3,333 and the proceeds were $3,330, for no taxable gain. The cost basis of the land is reduced by the $10,000 you claimed as cost basis for the strip, so the new cost basis is $90,000. That will possibly result in a larger gain later, if your father sells before he passes, but paying tax later is better than paying it now. Especially since you and your brother will inherit with a fully stepped up basis, it is to your advantage to reduce the basis rather than paying tax now.
(As an alternative suppose the proportional cost basis of the taken land was $3000. You would each report a $1000 basis, $3333 of proceeds, and $2333 of capital gains, and the basis of the land would be reduced to $97,000. That would result in more tax now but less tax if your father sells in the future. But if he passes and you inherit a fully stepped up basis, then you paid tax now but you don't get a future tax reduction because of the basis.)
