The .into method provides a second argument which is a callback to success and failure. You can use this to keep track of when all three have been called and act on their visibility all at once.
Javadoc: https://square.github.io/picasso/2.x/picasso/com/squareup/picasso/RequestCreator.html#into-android.widget.ImageView-com.squareup.picasso.Callback-