How should the share of the third partner, who did not pay the development amount, be distributed between the first and second partners—is it according to each one's capital share, or equally between them?

If the third partner refuses to join the investment project on the shared land, then what the other two partners agree upon regarding the share of the refusing partner is what matters. It is not necessary for his share to be divided according to the proportions of ownership in the land unless the third partner specifies a particular division for himself or allocates it exclusively to one of them. Drawing lots is preferable to appease hearts and avoid the accusation of bias.