Let me start by acknowledging today being 100 years to the end of world war 1. Hence, i'll be paying my respect to all the fallen heroes and innocent civilians who lost their lives during the war.
World war 3, if ever it happens, is going to be disasterous and it's mainly because war fare has evolved since the previous wars. If you take a look at historic materials on the previous wars, you'd realize that military war strategies and most importantly, weapons used back then are now outdated and classified as less effective in modern day battles. But still, imagine the amount of damage done with those old type of weapons. Now imagine the amount of destruction that will be caused with modern weapons. Let's take a quick look at some weapons comparison from the first and second world wars.
During the first world war (1913 -1918), one of the most advanced weapons used was the vickers machine gun produced by the British. It took a minimum of 6 men to operate the gun and had the capacity to fire about 450 - 500 rounds of bullets per minute. At the time, this was a very powerful weapon that caused a lot of damage and took the lives of thousands of soldiers. It took the life of an approximate 20+ million people to end the war.
World war 2 (1939 - 1945) on the hand saw the introduction of more advance weapons like the Atomic bomb, Me 626 Messerschmidt (the world's first fighter jet), the STG 44, Armored tanks, etc. These weapons were really destructive and had firing rates way higher than those of the first world war. They covered more grounds and could shoot targets farther than those of the previous wars. Over 60 million people (3% of the world) were killed during this period.
Today, we have even more advanced weapons than those used in the second world war. Some of todays weapons are less heavy with minimum men to handle, they also have higher firing rate and can cover more grounds like never before.There's the nuclear and hydrogen bombs, as well as other chemical weapons that are way advance and powerful than previous ones like the nerve gas used during the first world war. There's the all too familiar Ak47 which was introduced after the second world war. We have missiles which can be fired from one country to another, we have more advanced fighter jets and planes specifically made for dropping bombs from the sky, etc.
Drawing from the above, you'd notice that the second world war was more destructive than the first primarily because of the different types of weapons used. The more advance the weapons, the more destructive the war. Now, imagine the amount of destruction modern weapons can cause using the reference stated above. The world would barely survive if there was a third world war and full implementation of modern day weapons. Barely survive will mean was left of it. I personally believe that a third world war will be the end of civilization and probably the start of an apocalyptic earth.