visible property

1. How can we have same footer in multiple mashups??

     You can use a Master Mashup or Embed a Mashup in a Mashup

2. And how can I make a container invisible when I click one button & visible when i click another button? I didnt find visible property for container widget.

     I don't think that is possible for a container, you can I think collapse and expand it.

3. How to control "checkbox" of widgets using expression or something else!!

     If they are bindable, you can use an expression or a service or another widget and bind the value to it for control