I recently purchased a LG 25UM58 monitor as a third and I have had extreme trouble getting it to work, I'm using a regular HDMI cable into my graphics card but all I get is no signal. I have only had it work once out of no where but when turned off my computer and turned it on the next morning it no longer worked.

I'd really appreciate some help on this as I have no idea what is causing the problem.

 

Also never had any luck with 3 separate screens on a 'standard' GPU. Your best bet is to get extra hardware that's build for the job.
Right now, I've got a Blackmagic Extreme card for a third screen, but use that for video playback In Pro Tools only.
Matrox is the no.1 brand for multimonitor setups, it's used in dealing- & control rooms all over the world.
They're not cheap, but a their 'Dualhead2go' or 'Triplehead2go' boxes can be bought on eBay for much less:
https://www.ebay.com/sch/i.html?_fr....TRC2.A0.H0.Xmatrox.TRS0&_nkw=matrox&_sacat=0

 

Maybe your HDMI cable is too long. Try with a 1-2m one [up to like 6-7 feet]. The shorter - the better. The more expensive - the better, too.

HDMI is really troublesome, especially with older TV/monitors. I hate it. So many people have troubles with it. DisplayPort is much better and DVI is still the best, but on its way out for some [extremely stupid] reason and good old VGA. These are two that really work, always.

 

Which video card do you have? Which connectors are being used for each of the 3 monitors? Have you updated to the latest video drivers?