will subscriber be available, if publisher or distributor is down?
There are locks taken on the subscriber during data transfer which can cause blocking on the subscriber - so you can definitely encounter issues there (and there are options to modify this behaviour at the risk of not being able to rollback failed transactions). 

However there aren't any other magical locks or connections between the servers. 

But also when the publisher/distributor go down I've never seen the insert also fail to go down and not release the locks (aside from standard rollback time). I'd hesitate to say it wasn't possible but it would be abnormal and I haven't seen it. 

The much more common scenario is that the subscriber goes down, log records accumulate in the publisher, fill the disk and your publisher goes down. A large log file and monitoring of backups, space, and the replication itself are essential.