  Unsharp measurements play an increasingly important role in quantum
information theory. In this paper, we study a three-party
prepare-transform-measure experiment with unsharp measurements based on $ 3
\rightarrow 1 $ sequential random access codes (RACs). We derive optimal
trade-off between the two correlation witnesses in $ 3 \rightarrow 1 $
sequential quantum random access codes (QRACs), and use the result to complete
the self-testing of quantum preparations, instruments and measurements for
three sequential parties. We also give the upper and lower bounds of the
sharpness parameter to complete the robustness analysis of the self-testing
scheme. In addition, we find that classical correlation witness violation based
on $3 \rightarrow 1 $ sequential RACs cannot be obtained by both correlation
witnesses simultaneously. This means that if the second party uses strong
unsharp measurements to overcome the classical upper bound, the third party
cannot do so even with sharp measurements. Finally, we give the analysis and
comparison of the random number generation efficiency under different sharpness
parameters based on the determinant value, $2 \rightarrow 1 $ and $3
\rightarrow 1 $ QRACs separately. This letter sheds new light on generating
random numbers among multi-party in semi-device independent framework.
