Yes you should get a cooling fan for your esc.if you plan to run on 3s. You should also get a motor heat sink to keep your motor cool.. There are many third party ones you could go for those if you want.

Reminder set your esc on Lipo mode (green esc light).

To clarify about the heat sink I recommend earlier is that you do not need to modify it to fit a 2WD Traxxas Rustler. It just snaps on pretty easily no modifications needed. Works as expected. Can be used even with a wheelie bar installed. heat sink link