Proof of two congruence conjectures of Z.-W. Sun

In this paper, we mainly prove two congruence conjecture of Z.-W. Sun. Let $p\equiv3\pmod 4$ be a prime. Then $$\sum_{k=0}^{p-1}\frac{\binom{2k}k^2}{8^k}\equiv-\sum_{k=0}^{p-1}\frac{\binom{2k}k^2}{(-16)^k}\pmod{p^3}.$$ And for any odd prime $p$, if $p=x^2+y^2$ with $4|x-1, 2|y$, then $$\sum_{k=0}^{p-1}\frac{(k+1)\binom{2k}k^2}{8^k}+\sum_{k=0}^{(p-1)/2}\frac{(2k+1)\binom{2k}k^2}{(-16)^k}\equiv2\left(\frac{2}p\right)x\pmod{p^3}.$$