Could you give a couple of examples of where that is said?

It depends on what that statement is supposed to mean. Is it 3 days of actual combat during the whole war or 3 days of combat at a time? I’m not familiar with that statement I can see a couple ways it might be somewhat accurate.

The British used a 3 level trench system that consisted of a firing trench, a support trench and a reserve trench. Soldiers would get rotated through the different levels spending a little less time the closer they got to the front trench. Average time spent in the first trench is probably around 3 days. Most days would be spent repairing the trench, praying during bombardments and hiding from snipers. If you weren't in the trenches you were behind them some ways at “rest”. The only ways you saw actual combat was if you were unlucky enough to be in the trenches during a charge or in the wrong place when a small group of enemies snuck over for a raid.

If you’re interested, a good book to read is All Quiet on the Western Front. It’s not a true story but it’s based on the author’s experiences and is praised for accurately depicting the life of a solider in WWI. I heard somewhere that the Nazis banned it because it doesn't glorify war.

Also, here’s a link to an article that gives more detail about Life in the Trenches.