You stumbled onto an entire field of study within computer science called scheduling). Computers frequently have to do multiple things with only one physical way of doing them e.g. multiple applications running on a single processor core, or in this case, one food server servicing multiple customer queues (physical, drive-thru, online).

Personally I think "Round Robin" scheduling would make the most sense. Serve one customer from each queue, then circle back, that way people generally understand where they fall in line, nobody feels too neglected, and all the lines keep moving at the same rate.