Definitive way to tell
Settings, General, Model
If it's one of these, it's a 3rd gen:
The new iPad 16GB Black Wi-Fi:*MC705LL/A
The new iPad 32GB Black Wi-Fi:*MC706LL/A
The new iPad 64GB Black Wi-Fi:*MC707LL/A
The new iPad 16GB White Wi-Fi:*MD328LL/A
The new iPad 32GB White Wi-Fi:*MD329LL/A
The new iPad 64GB White Wi-Fi:*MD330LL/A
The new iPad 16GB Black*Wi-Fi + 4G LTE for AT&T:*MD366LL/A
The new iPad 32GB Black*Wi-Fi + 4G LTE for AT&T:*MD367LL/A
The new iPad 64GB Black*Wi-Fi + 4G LTE for AT&T:*MD368LL/A
The new iPad 16GB White Wi-Fi + 4G LTE for AT&T:*MD369LL/A
The new iPad 32GB White Wi-Fi + 4G LTE for AT&T:*MD370LL/A
The new iPad 64GB White Wi-Fi + 4G LTE for AT&T:*MD371LL/A
The new iPad 16GB Black*Wi-Fi + 4G LTE for Verizon:*MC733LL/A
The new iPad 32GB Black*Wi-Fi + 4G LTE for Verizon:*MC744LL/A
The new iPad 64GB Black*Wi-Fi + 4G LTE for Verizon:*MC756LL/A
The new iPad 16GB White Wi-Fi + 4G LTE for Verizon:*MD363LL/A
The new iPad 32GB White Wi-Fi + 4G LTE for Verizon:*MD364LL/A
The new iPad 64GB White Wi-Fi + 4G LTE for Verizon:*MD365LL/A