Same as becoming a third wheel but in the company of another who prefers to text/converse with someone else than you. Typical occurrence for 'besties' which one of them recently got engaged in a relationship.
A: Saw you riding train next to the popular chick, how'd it go?
B: Boring. I got Digitally third wheeled the whole ride.
(A: Digi-wha..?
B: a 'Digital Third Wheel' is- Same as becoming a third wheel but in the company of another who prefers to text/converse with someone else than you.
A: Dang that sucks..)