If Sensor and Area can be "sen" or "aa", then just use OR = "||"
string.Equals is a great method to compare strings.
string sensor = "sen";
string area = "aa";
string system = "sys";
var listSegment2 = from ls1 in SMDB.SegmentInfos
where string.Equals(ls1.Area, area, StringComparison.InvariantCultureIgnoreCase)
|| string.Equals(ls1.Sensor, sen, StringComparison.InvariantCultureIgnoreCase)
|| string.Equals(ls1.System, system, StringComparison.InvariantCultureIgnoreCase)
select ls1;
foreach (var segment in listSegment2)
{
Console.WriteLine("Segment - Area:" + segment.Area + ", Sensor:" + segment.Sensor);
}