Nobody has had a guess at this, so allow me.
It sounds like a kernel or hardware problem. One thing you could try is to download Slackware's latest 'huge' kernel. Packages are in tar.xz format. My local mirror is
ftp://ftp.heanet.ie/mirrors/slackwar...2/slackware64/
You need the kernel-huge & kernel-modules txz packages which are tar.xz and located in the 'a' directory. Open them in a tmpdir, and copy it all into place & set it as an extra kernel. That has everything you mightn't have in your kernel, and will eliminate the kernel. Then you're down to hardware issues. I imagine you need to find someone who has done 3 GPUs running.
It may also be power supply limitations. GPUs suck power. How big is your power supply? There's a 'power good' line from the psu which monitors voltage. 5V or 3.3V could be low if overloaded. Or the tracking on the motherboard could be dropping too much voltage.