Alright, took 5 minutes to see what I could see. First up, no glaring GMB specific issues. No duplicates, the phone number and address look clean. NAP looks pretty good, though I did see an extra address banging around: 7127 S 400 W 3. Might be worth looking into a little to see how important the sites with that address are, and if it's worth spending any time to address some of them.
Reviews are certainly a factor, but just one of a whole ton... though based on your moz citation check, I'm guessing you've been using their service, so you probably already know you've got more than one factor to pay attention to.
Overall, I think the main things that are holding you back are organic signals. If you check out the page below the 3 pack for "Signs Utah" (I tried one or two other searches too) your site shows up near the bottom of the page, well below the competition's sites. Organic site ranking factors influence 3-pack.
Page load speed wasn't great, might want to check that out. There's no schema, though that doesn't have huge correlation with ranking... still worth setting up, it takes like 20 minutes. You have a blank h1 tag on the home page, and the h2 tag doesn't include any geographic information. I'd be including city as well in title/h1 tags. You might want to stick a
map on the site too. There's a correlation between text and ranking as well... you might want to write more copy for the home page especially. It looks like you hit a few alt title tags, but it's not a thorough job. Personally, if you're going to take a look at the website, I'd consider making some changes to the design from a conversion standpoint too, but that's neither here nor there. You should do a thorough audit of the website and really bring it up to speed. There's a whole grip of stuff that you can check out, but for a quick primer, Casey's stuff is always awesome. This one's old, but most of the on-site checklist stuff is still stuff worth checking. This is a good a place as any to start. Onsite optimization is something I'm still coming up to speed on too, so others might have other resources and tools to point you to.
https://moz.com/blog/ultimate-local-seo-audit
Your backlink profile is pretty weak too (on Majestic, I'm seeing 35 backlinks from 17 sites). Fast Signs has a ton, though their GMB web link is 404'd? Whoops. If you're looking for ways to brainstorm, I'd check out t
his epic roundtable from last year. I'd also check out
this, though I haven't spent as much time doing anything out of that resources, so... consider it food for thought more than a hearty recommendation.
Either way, looks like you've done good work on the GMB site of things. Time to hit the organic too.
