L1 and L2 Visa Extension Concerns

Hi. My husband has an L1 and I have an L2 VISA and both live and work in NYC. We have also applied for our Green cards. The visas hit their 3 year mark next year so I know we need to apply to extend (assuming the Green cards don't come through by then). 1) Is it done online/paper filing (i.e. or do we need to go back to the US embassy in London 2) when should we apply and 3) Is their any travel restrictions while its being process? (I have a family wedding next year so I will need the passport)