At the end of section 3, the paragraph:

The value(s) advertised by Alt-Svc can be used by clients to open a new connection to one or more alternative services immediately, or simultaneously with subsequent requests on the same connection.

says that several alternative services can be used simultaneously by a client.

However, section 2.4 Using Alternative Services, recommend choosing one alternative service and using only one alternative service (except during the transition time).

I think we should align the two by rephrasing the sentence in 3.