There are a couple options for who will be involved in WW3.
Countries that will 100% be in WW3 -
The United States of America. The US can never resist a good fight, and is as of right now the nation with the tensest relationships. Almost every one of the US's alliances has been tainted with Trump's actions, and nearly every country sees the US as a joke, at least in terms of leadership. If the US was to go to war with another nation, most of it's allies would most likely be because of a common enemy. If the US was to go to war with North Korea, The British would most likely join since they are enemies with North Korea too, instead of because they are allies with the US. That being said, The US still has the strongest army in the world by far, and could most likely stand alone if they were only at war with one or two, maybe even three other nations.
Countries that will most likely be in WW3-
Russia. Russia has been making very big moves in the grand scheme of things. They've been getting into the US Democratic system, and if the US somehow makes it through Trump's presidency, the next president will most likely set extreme sanctions on Russia and possibly go to war if they continued to interfere with American politics. Russia is also a communist hub, meaning many capitalist nations in the world aren't too nice-nice with Russia and might want to join in if a war breaks out.
North Korea. North Korea has been getting further and further every day with their nuclear program, and no matter how many times they promise to stop, they continue. Eventually, they may end up nuking themselves on accident or starting a war with another nation.
China. China has strong alliances with both of the countries above, being tightly connected and not enemies of North Korea, and using the communist economic system for the most part, meaning they might join Russia if they are in a war. As of right now, China's alliance with the US is held on by a thread, and mostly by economic ties, since the US purchases many of the products China makes.
Countries that might be in WW3-
The EU. While not technically a country, many large countries in the EU such as England and Germany may join in a war if one of their allies are in one, but it is unlikely they will start one themselves since they've been mostly dormant and non-threatening (at least not as much as the US or Russia) on a worldwide scale
The Middle East. While not technically a country again, some countries in the Middle east are either harboring (maybe not by choice) terrorism or nuclear weapons, both things the US doesn't like at all. If either of these were to pass the line, the US might jump in and start a war.
And that's basically it. WW3 could be right around the corner for all we know, especially with the current political climate, and the US will most likely be right in the middle of it.