Which way of writing a conditional test is the most readable?
I would suggest to add two more intermediate variables:

var allTriggersAvailable = meterFlag &amp;&amp; workFlag &amp;&amp; timeFlag;
var anyTriggerAvailable = meterFlag || workFlag || timeFlag;

if (allTriggersAvailable || (!IsAllTriggersRequired &amp;&amp; anyTriggerAvailable))
{
    result.Add(new TodayTaskTest { Code = item1.Code, Id = item1.Id, Title = item1.Title });
}