That's true, IF passengers are interested in sharing a ride with two other passengers (and if they're interested in trading cost savings for the delays involved in that).

I kind of doubt that there will ever be a day when rides have an average of 3 passengers. Figure that if your base fare is $20, the first person you share with saves you $10 (or maybe $9 or something if you have to shore up the profits of the hailing app). Second person you share with saves you... $3. Third person saves you $2.

The returns to the passengers for additional passengers falls off fast, and the costs to the passengers (in terms of time, of discomfort, etc.) are linear or even more than linear. So probably not three passengers.

But two passengers is possible and would make for a real network effect... if ultimately there's enough demand for shared rides at all. There might be! Hard to say.