An EUI is not required in V3 if your gateway doesn’t have one.
The Things Kickstarter Gateway doesn’t have an EUI, so you can just leave the field empty.
The EUI of The Things Indoor Gateway can be derived from the code on the sticker on the back of the gateway, but you may want to wait with registering TTIGs on V3 (see Connecting TTIG to TTN V3 - (NOT YET) - #58 by ohlhues).