You can check the final asm trivially online, e.g., https://run.dlang.io/is/e0c2Ly (click the ASM button). You'll see that your params are in R8, RDX and RCX (reversed order as mentioned earlier).
You're not using naked asm; this entails a prologue (spilling the
params to stack etc.). Additionally, LDC doesn't really like
accessing params and locals in DMD-style inline asm, see
https://github.com/ldc-developers/ldc/issues/2854.
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