You need a lot more than 3X solar capacity to deal with night time, which Coal has no issue with. You need some kind of storage (battery? Pumped hydro?) and that is expensive.
Storage is expensive for now. Just a few years ago, solar was mocked as being ridiculous and having no future due to terrible efficiency and high costs. Now that manufacturing is well established and people know how to set it all up, it's the cheapest source of energy.
Batteries beyond the scale of a handheld device only started getting massively manufactured and invested in fairly recently as well. Once it's obvious that it's possible to build megabatteries that can power towns, everyone will want in on the market and prices will go down.
Eventually. Right now there's the idea that batteries=lithium, and most manufacturers aren't even considering other materials. There are much cheaper materials out there that can make batteries, especially when looking at devices of massive scale.
