Hi,
The traffic is not being source-nat.
Could you try pinging from a host instead of the SRX interface and capture the show security flow session.
You could also perhaps specify the LAN subnet [10.0.3.x/x] to match the source-address for the source-nat.
Also, the traffic is matching self-traffic-policy:
Session ID: 20765, Policy name: self-traffic-policy/1, Timeout: 42, Valid In: 10.0.3.1/1 --> 66.117.151.5/15860;icmp, If: .local..0, Pkts: 1, Bytes: 84 Out: 66.117.151.5/15860 --> 10.0.3.1/1;icmp, If: vlan.2, Pkts: 0, Bytes: 0
Cheers,
Ashvin