AirPort Express - Create Network or Extend Network?
Choosing "Extend Network" will greatly reduce the bandwidth (read: download speeds) available to your devices connected via Wifi, since one router has to relay data to the next wirelessly.

Apple has a good support article on extending your network, the various choices and the impact it has. "In the case of a wirelessly extended network, throughput may be reduced to less than 60 percent of that of a single device.": https://support.apple.com/en-us/HT202056

Since, as you say, you cannot interconnect your three Airports via Ethernet, the only choice is to extend. You can only "Create a New Network" if your Airport can reach your Internet router/modem via Ethernet. 

If you have an ethernet connection to all three Airport routers, then creating a new network, even with the same name will give you better bandwidth, since the three routers should configure itself to be on different bands (or you could manually configure separate bands with the Airport utility as well).