I don't think you should worry too much, but if you want to install 3x Gen 4 drives then I think your GPU will be reduced to x8, whichever slots you use.
I (think) that your mobo has 20 lanes. So, I suggests the easiest way to be sure is install your drives to M.2_1 - M.2_3 inclusive and then try M.2_2 - M.2_4 inclusive. If you pick a drive and do a basic install of Windows to test the config in each case (let's say M.2_2), then it doesn't take very long to find out using something like GPU-Z. You should be all done in 90 mins or less easily. The only pain is potentially removing the GPU to get easy access to the M.2 slots (I don't have you mobo, so not certain if that's an issue). Just don't forget to backup any important files if you are going to be wiping drives and reinstalling Windows!
Anyway, the interesting thing here is that the GPU performance difference you get between x8 (worst case) and x16 is pretty small. What I'm saying is that even if you do get x8, it won't be very noticeable. For example, see the testing undertaken here:
I would just go down the line in order. M_1, _2, _3. All will be at full speed. Pretty sure there are 4x pci-e from cpu and 12x from pch. Anyways, hope this helps.
Hey I use the strix 690-a which looks to be the same board with ddr4. I don't think you have anything to worry about even if you load all 4 M2 slots.
The app in the screenshot is hwinfo (free)