Does secp256r1 or secp256k1 use a=-3?
The equation for secp256k1 is $y^2 = x^3 + 7$, so that $a = 0$ and $b = 7$.

The equation for secp256r1, also known as NIST P-256, is $y^2 = x^3 - 3x + b$, where $b$ is 41058363725152142129326129780047268409114441015993725554835256314039467401291; here $a \equiv -3 \equiv p - 3 \pmod p$ where $p = 2^{256} - 2^{224} + 2^{192} + 2^{96} - 1$ is the characteristic of the field underlying secp256r1.

If you change the curve parameters, you get a different curve.