A simple adapter to put on the Ender 3 Pro to use MG12 linear rails, either C or H should work, I tried with MGN12C. Mounts using the M5 screws on the bed. Use M5 threaded inserts on the large holes before mounting.

Mounting instructions: Print this model x2 using Voron Specs and ABS or ASA material Print linear rail alignment tools about 6-8 in total. Or 3-4 per side. Insert heat inserts (M5) Need M3 screws of various sizes as well as T-nuts

Align the rails using the tools and screw them into the aluminium extrusion using M3 screws and T-nuts. Do not tighten too much, yet. Mount the adapters here to the rails and then the aluminium that holds the bed using the M5 original screws. Then test if it slides freely, else adjust a little as needing on the screws holding the rails in place to make sure it aligns itself nicely.

Inspired by this one but remodelled it to suite MGN12C & H rails. Uses the same license, too. https://www.thingiverse.com/thing:4141819 License: https://creativecommons.org/licenses/by-nc/4.0/