If your transfers from outside the EC2 cloud to the new m3 instances fail, then it might be due to jumbo frames configured by default on this new type of instance.
I have an external machine which also uses jumbo frames and during the tcp handshake the machines advertise maximum segment sizes (MSS) of 8960 and 8961bytes respectively.
I’m not really sure if this is Amazon’s fault or a misconfiguration on my side but the fix is to lower the MTU back to 1500 on the instance. Sure, intra-area transfers might be slower but at least I can copy stuff from outside.
ip link set dev eth0 mtu 1500