Unlikely that a pot grown specimen will get big enough to cover 3 metres however big the pot, and any pot you use should be 2 feet deep by 12 inches wide as a minimum.

Cotoneaster does live with poor nutrents and cramped roots though, its roots are fairly shallow, Don't know about 3 meters, but little protection from easterly winds and a pot/trough up to 1m wide could work. fairly robust plants I have found growing them on a hill in N Wales.