In my experience, doing this can:

* Reduce performance (usually by half on the single module as it is running in single-channel mode rather than dual-channel)
* Reduce reliability (largely because you invalidate the XMP timings, but depends on motherboard and BIOS, and is the same when you mix&match modules even if you install them in pairs); rule of thumb *should* be to set timings for the slowest module. If yours were the same speed/timings on the label, that will help a lot, and you can probably write down what the timings are that have been working for you so far and just manually set them that way after installing the third module.

Most higher-end boards shouldn't suffer any serious errors as a result but you may see some really strange performance hiccups depending on whether your app is using memory pages in the single-channel or dual-channel regions.

As always, though, trust but verify. I would pull the new third module back out where you're sitting at your original 32GB and run an overnight Prime95 stress test with settings per https://www.reddit.com/r/overclockin...ram_stability/. Then, if that passes OK (and it should lol), reinstall the extra module, change your Prime95 settings for the 48GB setup and run it overnight again. If it passes overnight, you should be good to go. If you have issues, you can try backing speeds down a bit or reducing timings, but a cursory search for recommendations for that didn't yield much beyond conjecture and "don't do that" admonitions. :P

And if you see strange stuttering or artifacting in the future, pull that third module back out for a while and see if it goes away. Memory can be really tricky when you have imbalanced timings or voltages and such.