App Design Process Question

When designing an app that will be used on phones, tablets and computers - do you design the layout for all three at the same time or just focus on one and then go back and make use of Context views to do the layout for the other two?

I’ve been focusing all my design to look good on the phone but know I need to go back and tweak it to look equally good for tablets and browsers.

I’d love to hear what method you employ and why you chose that way.