What happens when all secondary read only nodes are powered off?
Make sure you include the primary in the read_only_routing_list

ALTER AVAILABILITY GROUP  aggroup
MODIFY REPLICA ON 'node1'
WITH
(
PRIMARY_ROLE
(
READ_ONLY_ROUTING_LIST= ('node2','node3','node1')
)
); 


Do the same for the other 2 nodes. and you can test using this query 

SELECT    AVGSrc.replica_server_name AS SourceReplica       
        , AVGRepl.replica_server_name AS ReadOnlyReplica
        , AVGRepl.read_only_routing_url AS RoutingURL
        , AVGRL.routing_priority AS RoutingPriority
FROM sys.availability_read_only_routing_lists AVGRL
INNER JOIN sys.availability_replicas AVGSrc ON AVGRL.replica_id = AVGSrc.replica_id
INNER JOIN sys.availability_replicas AVGRepl ON AVGRL.read_only_replica_id = AVGRepl.replica_id
INNER JOIN sys.availability_groups AV ON AV.group_id = AVGSrc.group_id
ORDER BY SourceReplica