We are not in WW3 yet. However I can see it happening within the next 5 to 10 years. Here are the current situations that are happening now.
1. North Korea has recently become more of a threat do to their new missile program. The continue to make threats to the US.
2. Russia marched into Crimea over a year ago and they took over the country within hours and is a violation of international law. It is the equivalent of when Hitler took over Poland which is what started WW2.
3. Iran has agreed to a nuclear deal with the United States. Recently, Iran has stated that the United States is not allowed to inspect their nuclear sites which was part of the deal that was made. This means that they have something to hide. Potential nuclear weapons of mass destruction.
4. ISIS regime is spreading over the middle east very rapidly. Killing anyone that is against them or does not agree with them. More dangerous than Al-Qaeda despite 9/11.
5. Israel now does not trust the US as much as they used to. The reason for this is because the US made the Nuclear Deal with Iran.
So the question remains, how will WW3 break out?
This is my opinion of what can and most likely will happen, so take it with a grain of salt.
I firmly believe that the United States will go back to the middle east and engage in another war to eradicate ISIS. After that North Korea launches a missile to target the US because we have been weakened from the war with ISIS. We then invade North Korea from the South Korean border also known as the DMZ. We will then be in an all out war with North Korea. China, our current ally in trade would then back North Korea. China is a very close ally to North Korea and would back them before the US. Now the United States has to fight North Korea and China. Then US would need ask the United Kingdom, Israel, and Canada for the help. Israel agrees to help despite the nuclear made with Iran. Russia would come out of nowhere and fight with China and North Korea. But wait! What about Iran? They would side with North Korea, Russia, and China and use nuclear weapons against us. Iran was making Nuclear Weapons the entire time which is why they would not let the US inspect their nuclear sites.
Conclusion: United States, Israel, United Kingdom, and Canada VS. Russia, China, North Korea, and Iran