I think you are confusing the discrete case with the continuous case.
Ax:3=v qx + v^2 * px * q(x+1) + v^3 * 2px (Note payment is made at end of the
3rd
year
regardless of whether he lives or dies
in the
third year.)
You are correct that the term plus the pure endowment equals the endowment.
Ax1:3+Ax:13= v qx + v^2 * px * q(x+1) + v^3 * 2px * q(x+2) + v^3 * 3px
3px = 2px * p(x+2)
Ax1:3+Ax:13= v qx + v^2 * px * q(x+1) + v^3 * 2px * q(x+2) + v^3 * 2px * p(x+2)
Ax1:3+Ax:13= v qx + v^2 * px * q(x+1) + v^3 * 2px * (q(x+2) + p(x+2))
Ax1:3+Ax:13= v qx + v^2 * px * q(x+1) + v^3 * 2px
Regarding the NSP for a whole life insurance payable at the end
of the year of death given constant force:
d=delta, u=force of mort.
u/(u+d) is the NSP for the continuous case.
The NSP for the discrete case is derived as follows:
NSP = e^-d * (1-e^-u) + e^-2d * e^-u * (1-e^-u) + e^-3d * e^-2u * (1-e^-u). . .
.. . . .
NSP = [e^-d * (1-e^-u)] * [1+ e^-d * e^-u + e^-2d * e^-2u. . . . . . . . . .
The second portion is a infinite geometric series.
NSP = [e^-d * (1-e^-u)] * [1/(1- e^-d * e^-u)]